148 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



is predominantly anteroposterior (Q, R) ; 2-3 days later susceptibility of 

 the posterior region has increased so that anterior and posterior arms of 

 the gradient are almost equal (S, T). No essential difference in the re- 



FiG. 51, A-T. — Differential susceptibility in development of the lamprey Entosphemis 

 appendix; disintegrated regions indicated by dotted bounding-lines; and the chief directions of 

 progress of disintegration, by arrows. A, unfertilized egg; B, eight-cell stage; C, about thirty- 

 two cells; D, later cleavage; E, late cleavage or early blastula; F, G, two stages of disintegration 

 in late blastula; H-J, three stages of disintegration at beginning of gastrulation; A', L, later 

 gastrula; M, N, neural groove stage, disintegration progressing posteriorly, but original 

 apicobasal gradient evident in later disintegration (N); 0, P, susceptibility decreasing posteri- 

 orly from the elongating head along the nervous system and an increased susceptibility along 

 the segmental plate; Q, R, at hatching, susceptibility decreasing posteriorly along the nervous 

 system, with secondary region of high susceptibility posteriorly; S, T, 2-3 days after hatching, 

 anterior and posterior arms of the gradient almost equal (after Hyman, 19266). 



