SECT. 3] AND ORGANISATION 593 



blastula before movement begins the disintegration gradient is dis- 

 tinct, but the difficuky in identifying the animal pole and embryonic 

 axis makes it impossible to demonstrate that the gradient coincides 

 with the axis". But "in later free swimming stages of the blastula 

 the direction of movement with the apical region, the animal pole, 

 in advance, and before gastrulation the elongation of the embryo 

 in the direction of the axis and the increasing thickness of the cellular 

 layer toward the vegetative pole render orientation possible at a 

 glance. In these stages the disintegration gradient is very distinct. 

 It begins at the apical end and proceeds with a definite course along 

 the embryonic axis, ending in the region of the vegetative pole 

 where the gastrular invagination will occur". The susceptibility 

 gradient in the gastrula of the starfish Child found to be very similar 

 to that in the blastula, being greatest in the region of the apex and 

 least in the region of the blastopore. Moreover, the gradient in the 

 archenteron wall was exactly the same as that in the body-wall, 

 the apical end of the endodermal invagination being the region of 

 greatest susceptibility. After this time the gradients become less and 

 less distinct until by the bipennaria larva stage they have faded away 

 almost entirely. 



Hyman extended in 1 9 1 6 the observations of Child on polychaete 

 eggs to those of a microdrilus oligochaete, Tubifex tubifex. She found 

 that, in the stage when the embryo has begun to elongate, its posterior 

 region was the most susceptible to cyanide, and the susceptibility 

 decreased as one passed forwards. In later stages the head end 

 became more susceptible, and finally exceeded greatly the tail end 

 in susceptibility, so that at hatching it was much the most easily 

 disintegrated region. Thus the posterior region of high physiological 

 level which is characteristic of the adult annelid arises very early in 

 development. After hatching, a worm placed in cyanide disintegrates 

 first at the head end, later at the tail end, so that the two waves of 

 disintegration reach and fuse at a point posterior to the middle of 

 the worm's length. Figs. 98 and 99, taken from Hyman's paper, show 

 the degeneration of embryos of different stages in cyanide. 



Child and his associates frequently correlated their susceptibility 

 gradients with gradients of electric potential. Hyman & Bellamy in 

 1922 gave a full account of the work on this subject with a critical 

 discussion, and at the same time reported their results for frog 

 embryos. Hyde had previously found the heads of recently hatched 



N EI 38 



