ISO 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



embryo an anteroposterior gradient appears, corresponding to the polar 

 axis of the embryo (M, N). At a somewhat later stage, before closure of 

 the germ ring, a second region of high susceptibility appears at the pos- 

 terior end of the embryonic shield and persists in the posterior end of the 

 embryo {O, P). As particular organs — the eyes, the heart, etc. — are local- 



FiG. 52, A-P. — Differential susceptibility in early teleost development; regions and prog- 

 ress of disintegration indicated as in Fig. 51. A-F, Taulogohibrns adspersus; G-I, Fundidus 

 heteroclitus; J~P, Gadus morrhua (after Hyman, 192 1). 



ized and develop, they appear as regions of high susceptibility; in the em- 

 bryonic heart susceptibility decreases from the venous end. 



It is suggested by Hyman that the decrease of susceptibility from the 

 center of the early blastoderm peripherally in Tautogolahrns (Fig. 52, ^) 

 indicates a more primitive type of development with developmental ac- 

 tivity centered about the animal pole; the early appearance of higher 

 susceptibility peripherally in the blastoderm of Gadus is regarded as as- 

 sociated with the earlier appearance and more definite development of 

 the germ ring in this species. In any case, the relation of differential sus- 



