The Cell in Development and Inheritance

the ^gZ', and perhaps correlated The egg is surrounded by a very thick with this is the fact that several "-.embrane. perforated at m by the funnel- shaped micropyle; below the latter lies the spermatozoa enter the ^gg, though egg-nucleus in 'the peri-vitelline layer of pro- only one is concerned with the topiasm ; M the polar bodies, actual process of fertiUzation. The plant-ovum, w^hich is usually know^n as the oospJicrc ( Figs. 64, 107), shows the same general features as that of animals, being a relatively large, quiescent, rounded cell containing a large nucleus. It never, however, attains the dimensions or the complexity of struc- ture shown in many animal eggs, since it always remains attached to the maternal structures, by which it is provided with food and invested wdth protective envelopes. It is therefore naked, as a rule, and is not heavily laden with reserve food-matters such as the deutoplasm of animal ova. A vitelline membrane is, however, often formed soon after fertilization, as in echinoderms. The most interesting feature of the plant-ovum is the fact that it often contains phistids (leuco- plasts or chromatophores) which, by their division, give rise to tho.se of the embryonic cells. These sometimes have the ionii ot typical chromatophores containing pyrenoids, as in Volvox and many other Algse (Fig. 64). In the higher forms (archegoniate plants), according to the researches of Schmitz and Schimper, the ^gg contains numer- ous minute colourless 'Meucoplasts," which afterward develop into green chromatophores or into the starch-building amyloplasts. This is a point of great theoretical interest ; for the researches of Schmitz, Schimper, and others have rendered it highly probable that these

plastids are persistent niorpholo^c^ncal bodies that arise only by the division of preexisting bodies of the same kind, and hence may be traced continuously from one generation to another through the A. Ovum (oosphere) containing a large central nucleus and a peripheral layer of chromato- phores ; /. pyrenoid. B. Spermatozoid ; c.v. contractile vacuoles ; e. " eye-spot " (chromoplastid) ; /. pyrenoid. C. Spermatozoid stained to show the nucleus («).

germ-cells. In the lower plants (Algae) they may occur in both germ- cells ; in the higher forms they are found in the female alone, and in such cases the plastids of the embryonic body are of purely maternal origin. Although spermatozoa were among the first of animal cells ob- served by the microscope, their real nature was not determined for more than two hundred years after their discovery. Our modern knowledge of the subject may be dated from the year 1841, when Kolliker proved that they were not parasitic animalcules, as the early observers supposed, but the products of cells preexisting in the parent body. Kolliker, however, did not identify them as cells, but believed them to be of purely nuclear origin. We owe to Schweigger- Seidel and La Valette St. George the ]:)roof, simultaneously brought forward by these authors in 1865,^ that the spermatozoon is a com- plete cell, consisting of nucleus and cytoplasm, and hence of the same morphological nature as the ovum. It is of extraordinary minute- ness, being in many cases less than t;ooVoo' ^^^ bulk of the ovum.^

2 In the sea-urchin, Toxopiieus/es, I estimate its bulk as being between 4Tnnro7 ^^^ 5 ^ q^q ^ ,) the volume of the ovum. The inequality is in many cases very much greater. Its precise study is therefore difficult, and it is not surprisin^^ that our knowledge of its structure and origin is still far from complete. In its more usual form the animal spermatozoon resembles a minute, elongated tadpole, which swims very actively about by the vibrations of a long, slender tail morpho- logically comparable with a single cilium or flagellum. Such a spermatozoon con- sists typically of four parts, as shown in Fig. 65 : —

1. The nucleus, which forms the main portion of the **head," and consists of a very dense and usually homogeneous mass of chromatin staining with great intensity with the so-called "nuclear dyes" {e.g. haematoxylin or the basic tar-colours such as methyl-green). It is surrounded by a very thin cytoplasmic envelope. 2. An apical body, or acrosome, lying at the front end of the head, sometimes very minute, sometimes almost as large as the nucleus, and in some cases terminating in a sharp spur by means of which the spermatozoon bores its way into the ovum.

3. The middle-piece, or connecting piece, a larger cytoplasmic body lying behind the head and giving attachment to the tail, from which it is not always dis- tinctly marked off. This body shows the same staining-reactions as the acrosome, having an especial affinity for " plasma- stains " (acid fuchsin, etc.). At its front end it is in some forms (mammals) sepa- rated from the nucleus by a short clear region, the neck. Like the acrosome, the middle-piece is in some cases derived from an " archoplasmic " mass, representing an attraction-sphere {Lumbricus) or a portion of the Nebenkern (insects), and it contains, or according to some authors actually arises from, the centrosome (salamander, mammals, insects, etc.).

4. The tail, or flagellum, in part, at least, a cytoplasmic product developed in connection with the centrosome and " archoplasm " (attraction-sphere or "Nebenkern") of the mother-cell. Tt consists of a fibrillatcd axial filauioit surrounded by a cytoplasmic envelope, and in certain cases (Amjihibia) bears on one side a fin-like undulat- ing membrane (Fig. 66). Toward the tip of the flagellum the enve- lope suddenlv disappears or becomes very thin, leaving a short cnd-piccc which by some authors is considered to consist of the naked axial filament. The a.xial filament may be traced through the middle-piece up to the head, at the base of which it usually termi-

Fig. 66. — Spermatozoa of fishes and Amphibia, [Ballowitz.] A. Sturgeon. //. Pike. ('. D. Leuciscus. E. Triton (anterior part). F. Triton (posterior part of flagelkim). G. Raja (anterior part), a. apical body; e. end-piece; /. flagellum ; k. end- knob; w. middle-piece ; //.nucleus; j. apical spur, nates in a minute body, single or double, known as the cnd-kiwb. Recent research has proved that the axial filament grows out from the spermatid-centrosome, the latter in some cases persisting as the end-knob (insects, mollusks, mammals), in other cases apparently enlarging to form the main body of the middle-piece (salamander). The tail-envelopes, on the other hand, arise either from the *'archo- plasm " of the Nebenkern (insects) together with a small amount of unmodified cytoplasm, or from the latter alone (salamander, rat).

From a physiological point of view we may arrange the parts of the spermatozoon under two categories as follows : I. The essential structures which play a direct part in fertilization. These are : — (b) The middle-piece, which either contains a formed centrosome or pair of centrosomes (end-knob), or is itself a meta- morphosed centrosome. This is probably to be regarded as the fertilizing element par excellence, since there is reason to believe that when introduced into the Q.gg it gives the stimulus to division.

2. The accessory structures, which play no direct part in fertilization, viz. : — {a) The apex or spur, by which the spermatozoon attaches itself to the ^gg or bores its way into it, and which also serves for the attachment of the spermatozoon to the nurse-cells or supporting cells of the testis. {b) The tail, a locomotor organ which carries the nucleus and centrosome, and, as it were, deposits them in the <tgg at the time of fertilization. There can be little doubt that the substance of the flagellum is contractile, and that its movements are of the same nature as those of ordinary cilia. Ballowitz's discovery of its fibril lated structure is therefore of great interest, as indicating its structural as well as physiological similarity to a muscle-fibre. The outgrowth of the axial filament from the centrosome is probably comparable to the formation of spindle-fibres or astral rays, a conclusion of especial interest in its relation to Van Beneden's theory of mitosis (p. lOO). Tailed spermatozoa conforming more or less nearly to the type just described are with few exceptions found throughout the Metazoa from the coelenterates up to man ; but they show a most surprising diversity in form and structure in different groups of animals, and the homologies between the different forms have not yet been fully determined. The simpler forms, for example, those of echinoderms and some of the fishes (Figs. 66 and 100), conform very nearly to the foregoing description. Fvery part of the spermatozoon may. how- ever, vary more or less widely from it (Figs. 66-6(S).

The head (nucleus) may be spherical, lance-shaped, rod-shaped, spirally twisted, hook-shaped, hood-shaped, or drawn out into a long filament ; and it is often divided into an anterior and a posterior piece of different staining-capacity, as is the case with many birds and mammals, but it is probable that the anterior of these may represent the acro- some. An interesting form of head is described by Wheeler ('97) in A (At the left). Beetle {Copris), partly macerated to show structure of flagellum ; it con- sists of a supporting fibre {s./.) and a fin-like envelope (/) ; n. nucleus; a. a. apical body divided into two parts (the posterior of these is perhaps a part of the nucleus). /A Insect {Calathus), with barbed head and fin-membrane. C. Bird (Phyllopucnste). D. Bird { Muscicapa), -.how'mg spiral structure; nucleus divided into two parts («i. «-) ; no distinct middle-piece, E. Bulfinch ; spiral membrane of head. F. Gull (Larus) with spiral middle-piece and apical knob. G. H. Giant spermatozoon and ordinary form of Tadorna. /. Ordinary form of the same stained, showing apex, nucleus, middle-piece and flagellum. J. " Vermiform spermatozoon " and, K. ordinary spermatozoon of the snail Paludina. L. Snake {Coluber), showing apical body (a), nucleus, greatly elongated middle-piece {in), and flagellum f /).

the spermatozoon of Myzostoma, where it is a greatly elon^^atcd fusiform body, passing insensibly into the tail without distinct middle- piece and containing a single series of chromatin-discs. The num- ber of these in M. glabrnm is 24, which is the somatic number of chromosomes in this species. In j\L cirrifcruvi the number of chromatin-discs is more than 60. Somewhat similar spermatozoa occur in the acoelous Turbellaria.^ The acrosome sometimes appears to be wanting, e.g. in some fishes (Fig. 66). When present, it is sometimes a minute rounded knob, sometimes a sharp stylet, and in some cases terminates in a sharp barbed spur by which the sperma- tozoon appears to penetrate the ovum {Triton). In the mammals it is sometimes very small (rat), sometimes very large (guinea-pig), and in some forms is surrounded by a cytoplasmic layer forming the "head-cap" (Figs. 6'^, ^6). It is sometimes divided into two distinct parts, a longer posterior piece and a knob-like anterior piece (insects, according to Ballowitz).

The middle-piece or connecting-piece shows a like diversity (Figs. 66-68). In many cases it is sharply differentiated from the flagellum, being sometimes nearly spherical, sometimes flattened like a cap against the nucleus, and sometimes forming a short cylinder of the same diameter as the nucleus, and hardly distinguishable from the latter until after staining (newt, earthworm). In other cases it is very long (reptiles, some mammals), and is scarcely distinguishable from the flagellum. In still others (birds, some mammals) it passes insensibly into the flagellum, and no sharply marked limit between them can be seen. In many of the mammals the long connecting- piece is separated from the head by a narrow " neck " in which the end-knobs lie, as described below.

Internally, the middle-piece consists of an axial filament and an envelope, both of which are continuous with those of the flagellum. In some cases the envelope shows a distinctly spiral structure, like that of the tail-envelope ; but this is not always visible. The most interesting part of the middle-piece is the *' end-knob " in which the axial filament terminates, at the base of the nucleus. In some cases this appears to be single. More commonly it consists of two or more minute bodies lying side by side (Fig. 6%, B, D).

The flagellum or tail is merely a locomotor organ which i)hiys no part in fertilization. It is, however, the most complex part of the spermatozoon, and shows a very great diversity in structure. Its most characteristic feature is the axial filament, which, as Ballowitz has shown, is composed of a large number of parallel fibrillx, like a muscle-fibre. This is surrounded by a cytoplasmic envelope, which sometimes shows a striated or spiral structure, and in which, or in

connection with which, may be developed secondary or accessory fila- ments and other structures. At the tip the axial filament may lose its envelope and thus give rise to the so-called "end-piece" (Retzius). In Trito)i, for example (Fig. 66, /• ), the envelope of the axial fila- ment ('* ])rincipal filament ") gives attachment to a remarkable fin-like - — - membrane, having a frilled or undulating free margin along which is developed a "mar- ginal filament." Toward the tip of the tail the fin, and finally the entire envelope, disapj)ears, leaving only the axial filament to form the end- piece. After maceration the envelope shows a conspicuous cross-striation, which perhaps indicates a spiral structure such as occurs in the mam- mals. The marginal filament, on the other hand, breaks up into numerous parallel fibrillar, while the axial fila- ment remains unaltered (Bal- lowitz).

A fin-membrane has also been observed in some insects and fishes, and has been as- serted to occur in UKinimals (man included). Later ob- servers have, however, failed to find the fin in mammals, and their observations indi- cate that the axial filament is merely surrounded by an envelope which sometimes Fig. 68. — Spermatozoa of mammals. {A-Fixom showS traCCS of the same Ballowitz.I . , , C.'^A\. {Vesper ago). Z^. The same, flagellum and which is SO COUSpicUOUS in

middle-piece or connecting-piece, showing end-knobs. 4-Uq COnnectin<^^-Diece In the lophus) showing details. F. Head of spermatozoon SkatC the tail has tWO hla- of the pig. G. Opossum (after staining). //.Double mcntS, both COmpOSCd of spermatozoa from the vas deferens of the opossum. n i /~i -n . i i somewhat similar structure occurs in the toad. In some beetles there is a fin-membrane attached to a stiff axial "supporting fibre "( F"io-. 6-], A). The membrane itself is here composed of four parallel fibres, which differ entirely from the supporting fibre in staining-capacity and in the fact that each of them may be further resolved into a large number of more elementary fibrillae.

A. B. C. Living amoeboid spermatozoa of the crustacean Polyp hem us. [Z.xcH.VKl D. E. Spermatozoa of crab, Droniia. P. Oi Et/insa, G. oi Maja. //. of hi.uhus. I. Spermatozoon of lobster, Homarus. [HERRlCK.] J. Spermatozoon of crab, Porcellana. [GROBBEN.] Many interesting details have necessarily been passed over in the foregoing account. One of these is the occurrence, in some mammals, birds. Amphibia (frog). and mollusks, of two kinds of spermatozoa in the same animal. In the birds and Amphibia the spermatozoa are of two sizes, but of the same form, the larger i)eing known as -giant spermatozoa^' (Fig. 67, G, II). In the gasteropod Paluiima the two kinds differ entirelv in structure, the smaller form being of the usual type and not unlike those of biVds. while the larger, or "vermiform," spermatozoa have a worm-like shape and bear a tuft of cilia at one end, somewhat like the spermatozoids of plants (Fig. 67, J, K). In this case only the smaller .spermatozoa are functional (von Brunn).

No less remarkable is the conjugation of spermatozoa in pairs (Fig. 68, //). which takes place in the I'as dtferens in the opossum (Selenka) and in some insects (Hallowitz. Auerbach). IJallowitz's researches ('95) on the double spermatozoa of beetles {Dyiisciiia:) prove that the union is not primary, but is the result of an actual conjugation of previously separate spermatozoa. Not merely two, but three or more spermatozoa may thus unite to form a •• spermatozeugma,"' which swims like a single spermatozoon. Whether the spermatozoa of such a group separate before fertilization is unknown; but Hallowitz has found the groups, after copulation, in the female receptaculum. and he believes that they may enter the egg in this form. The physiological meaning of the process is unknown.

The principal deviations from the flagellate type of spermatozoon occur among the arthropods and nematodes (Fig. 69). In many of these forms the spermatozoa have no flagellum, and in some cases they are actively amoeboid ; for example, in the daphnid PolypJicnuis ( Fig. 69, A, By C) as described by Ley dig and Zacharias. More commonly they are motionless like the ovum. In the chilognathous myriapods the spermatozoon has sometimes the form of a bi-convex lens {Poly- (hs}fii(s), sometimes the form of a hat or helmet having a double brim {Jiihis). In the latter c^se the nucleus is a solid disc at the base of the hat. In many decapod Crustacea the spermatozoon consists of a

cylindrical or conical body from one end of which radiate a num- ber of stiff spine-like processes. The nucleus lies near the base. In none of these cases has the centrosome been identified. In most of the flowering plants the male germ-cells are represented by two "generative nuclei," lying at the tip of the ]:)ollen tube (Fig. 106). On the other hand, in the cycads (Figs. 87, 108) and in a large number of the lower plants (pterido- y^. Mother-cells with reticular nuclei. /A Later phvtCS, Muscineae, and many

stage, with spermatozoids forming C Mature sper- ^th^j-s), the male gCrm-CCll is a matozoid (the elongate nucleus black). '' ^ *^ minute actively swimming cell, known as the spennatozoiel, which is closely analogous to the sper- matozoon. The spermatozoids are in general less highly differenti- ated than spermatozoa, and often show a distinct resemblance to the asexual swarmers or zoospores so common in the lower plants (Fi^^s. 70, 71). They differ in two respects from animal spermatozoa : first in possessing not one but tw^o or several flagella ; second, in the fact that these are attached as a rule not to the end of the cell, but on the side. In the lower forms plastids are present in the form of chromatophores, one of which may be dif- ferentiated into a red *' eye-spot," as in Volvox and Fiicus (Figs. 57, 71, A), and they may even contain contractile vacuoles '^^

( Volvox ) ; but both these structures are wanting in the higher forms. These con- sist only of a nucleus with a very small amount of cytoplasm, and have typically a spiral form. In Chai^a, where their structure and development have recently been carefully studied by Belajeff, the sperma- tozoids have an elon- gated spiral form with two long flagella at- tached near the pointed end, which is directed forward in swimming (Fig. 70). The main body of the spermatozoid is oc- cupied by a dense, apparently homoge- neous nucleus sur- rounded by a very delicate layer of cytoplasm. Behind the nucleus lies a granular mass of cytoplasm, forming one end of the cell, while in front is a slender cytoplasmic tip to which the flagella are attached. Nearly similar spermatozoids occur in the liverworts and mosses. In the ferns and other pteridophytes a somewhat different type occurs

(Figs. 71, %?)). Here the spermatozoid is twisted into a conical spiral and bears numerous cilia attached along the upper turns of the spire. The nucleus occupies the lower turns, and attached to them is a large spheroidal cyto])lasmic mass, which is cast off when the spermatozoid is set free or at the time it enters the archegonium. This, according to Strasburger, proba])ly corresi:)onds to the basal cytoplasmic mass of Cliara. The upper portion of the spire to which the ciHa are attached is composed of cytoplasm alone, as in CJiara. Ciliated spermatozoids, nearly similar in tyj^e to those of the higher cryptogams, have recently been discovered in the cycads by Hirase {Gingko), Ikeno (Cj'cas), and Webber (2'^?;;//^?). They are here hemispherical or pear-shaped bodies of relatively huge size (in Zamia upward of 250 /x in length), with a large nucleus filling most of the cell and a spiral band of cilia making from two to six turns about the smaller end (Figs. 8j, 108).

As will be shown farther on (p. 173), the ** anterior " cytoplasmic region of the spermatozoid, to which the cilia are attached, is probably the analogue of the middle-piece of the animal spermatozoon ; and the work of Belajeff, Strasburger, Ikeno, Hirase, Webber, and Shaw gives good ground for the conclusion that it has an essentially simi- lar mode of origin, though w-e are still unable to say exactly how far the comparison can be carried. The *' posterior " region of the sper- matozoid appears to correspond, broadly speaking, to the acrosome.

Both ova and sj^ermatozoa take their origin from cells known as primordial germ-cells, which become clearly distinguishable from the somatic cells at an early period of development, and are at ftrst exactly alike in the two sexes. W^hat determines their subsequent sexual differentiation is unknown save in a few special cases. From such data as we possess, there is very strong reason to believe that, with a few exceptions, the primordial germ-cells are sexually indifferent, i.e. neither male nor female, and that their transformation into ova or spermatozoa is not due to an inherent predisposition, but is a reac- tion to external stimulus. Most of the observations thus far made indicate that this stimulus is given by the character of the food, and that the determination of sex is therefore in the last analysis a prob- lem of nutrition. Thus Mrs. Treat ('73) found that if caterpillars were starved before entering the chrysalis state they gave rise to a preponderance of male imagoes, w^hile conversely those of the same brood that were highly fed produced an excess of females. Yung ('81) reached the same result in the case of Amphibia, highly fed tadpoles producing a great excess of females (in some cases as high as 92%) and underfed ones an excess of males. The same result, again, is

given by the interesting experiments of Nussbaum ('97) on the rotifer Hydatina, which is an especially favourable case since sex is here de- termined at a very early period, before tJic egg is laid, the eggs being of two sizes, of which the smaller give rise only to males, and the larger only to females. The earlier experiments of Maupas ('91 ) on this form seemed to show conclusively that the decisive factor was temperature acting on the parent organism, since in a high tempera- ture an excess of females produced male eggs, and in a low tempera- ture the reverse result ensued. Nussbaum shows, however, that this is not a direct effect of temperature, but an indirect one due to the greater birth-rate and the greater activity of the animals under a higher temperature, which result in a speedier exhaustion of food. Direct experiment shows that, under equal temperature-conditions, well-fed females produce a preponderance of female offspring, and vice versa, precisely as in the Lepidoptera and Amphibia. These cases show that sex may be determined by conditions of nutrition either affecting the embryo itself (Lepidoptera, Amphibia) long after the ^^^^^ is laid, or by similar conditions affecting the parent-organism and through it the ovarian ^'g%.

Nutrition is, however, not the only determining cause of sex, as is shown by the long-known case of the honey-bee. Here sex is deter- mined by fertilization, the males arising only from unfertilized eggs by parthenogenesis, while the fertilized eggs give rise exclusively to females, which develop into fertile forms (queens) or sterile forms (workers), according to the nature of the food. This is a very excep- tional case, yet here too it is the more highly fed larvae that produce fertile females. It is interesting to compare with this case that of the plant-lice or aphides. In these forms the summer broods, living under favourable conditions of nutrition, produce only females the eggs of which develop parthenogenetically. In the autunni, under less favourable conditions, males as well as females are produced ; and that this is due to the external conditions and not to a fixed cyclical change of the organism is proved by the fact that in the favourable environment of a greenhouse the production of females alone may continue for years. ^

We are not yet able to state whether there is any one causal ele- ment common to all known cases of sex-determination. The observa- tions cited above, as well as a multitude of others that cannot here be reviewed, render it certain, however, that sex as such is not inherited. What is inherited is the capacity to develop into either male or female, the actual result being determined by the combined effect of conditions external to the primordial germ-cell.