Oxygen supply, circulation and regeneration in hydroids

Morgan (1901, Regeneration) used grafting to argue that a head forms at the anterior end of a piece through an internal factor. He held that the cut itself, or an external condition at the free end, does not account for it.¹ He took his examples from hydra, and he noted that tubularia often develops heteromorphic structures under external influence. He still thought the same internal factors act in tubularia. Small stem pieces also formed partial structures, and a red-pigment hypothesis did not explain them. Morgan could only assume that a very small piece is predisposed to form a larger partial structure than a smaller whole one.² Child (1915) described similar behaviour. He wrote that pieces one or two millimetres long usually develop into hydranths with a very short stem, or into partial hydranths, and that the result depends on their condition.⁵

Loeb (1906, The Dynamics of Living Matter) described pigments in the hydroid as respiratory pigments. He compared their purpose to that of red blood corpuscles.³ After a stem piece is cut, he wrote, the opening at each end closes within about an hour. In his observations no piece regenerated unless circulation was established. He suggested that circulation supplies oxygen, and that a liberal oxygen supply seems an absolute prerequisite for regeneration and growth.⁴ In 1916 he listed Tubularia among animals showing inhibition or correlation phenomena like those he had described in the plant Bryophyllum.⁶

Lillie (1923, Protoplasmic Action and Nervous Action) gathered evidence that form-correlation often has an electrical basis. He cited work reporting that regenerating hydranth heads are negative to the stems. Thompson (On Growth and Form) reported another of Loeb's findings. Cells of Tubularia must be turgescent to grow, and this is possible only below a certain salt concentration. Thompson gave maximal growth at a salinity well below that of normal sea-water.⁸