Artificial parthenogenesis and the chemical theory of fertilization

Wilson (1896) discussed parthenogenesis in relation to sex determination. He called the honey-bee a very exceptional case. There, sex is set by fertilization: males arise only from unfertilized eggs, and fertilized eggs give only females¹. He compared the plant-lice, whose summer broods under good nutrition produce only females from parthenogenetic eggs. Males appear in autumn, and Wilson took this to be an effect of external conditions, because a greenhouse kept female-only production going for years¹. Loeb (1912) later proposed that parthenogenetic propagation may have come before sexual reproduction. On that view, outside forces once supplied what the spermatozoon now supplies⁵.

Morgan (1901) reported that non-nucleated pieces of sea-urchin egg do not segment, whether from fertilized or unfertilized eggs. A piece entered by a spermatozoon does segment and, as Boveri and Wilson showed, forms an embryo. Boveri concluded from crosses between species that the nucleus fixes the character of the larva. Morgan called this evidence open to question, since he and Seeliger found great variability in the larvae from whole eggs crossed between the same two species². Loeb (1906) set two findings side by side. Parthenogenesis shows development without a sperm nucleus, and merogony shows that a nucleus-free fragment can develop when fertilized³.

Loeb (1912) named his account the lysin theory. The impulse to develop is given by a lysin carried in the spermatozoon⁴. In artificial parthenogenesis a cytolytic substance replaces the natural lysin. Normal development usually needs a second, corrective influence, which a hypertonic solution can supply⁴. Loeb (1916) recorded the opposition the work first met. Hertwig had defined fertilization as the fusion of egg and sperm nuclei, and no such fusion occurs when no sperm enters. Loeb therefore held that the definition had to be abandoned⁶. He said the objection that the effect was confined to a few species failed, since it had been produced in plants and in animals up to the frog⁶.

Crile's bibliography (1926) lists Gray's measurements of the electrical conductivity of fertilized and unfertilized eggs. One title ties them to fertilization and artificial parthenogenesis⁷. Crile lists the titles without reporting Gray's results. Recent work treats the sperm as an electrically regulated cell. Milani (2015) notes that sperm mitochondria have high membrane potential, including in species where they are passed on⁸. Pinto and colleagues (2023) review reports that depolarized human sperm are linked to lower fertilization rates⁹.