262 



Basic Structure of Vertebrates 



UP\ ,NC 





m 





-v^^^M 





Fig. 215. Sections of an amphibian embryo at an early stage in the develop- 

 ment of the notochord and mesoderm. Semidiagrammatic. (A) Median longitudinal 

 section. (B) Transverse section near the middle of the longitudinal axis, (a) 

 Archenteron; (bp) blastopore; (ec) ectoderm; (en) endoderm; (mes) meso- 

 derm; (nc) notochord; (np) neural plate. (Courtesy, Neal and Band: "Chordate 

 Anatomy," Philadelphia, The Blakiston Company.) 



blastopore (Fig. 215A). Initiated in this way, the mesoderm extends 

 into the lateral and anterior regions of 

 the embryo partly by growth within 

 itself, partly by continued contributions 

 from the blastoporal growth zone, and 

 possibly augmented by the detachment 

 of cells from neighboring surfaces of the 

 endoderm. 



The mesoderm of Amphioxus is seg- 

 mented at the time of its detachment 

 from the primary gastrular layers, and 

 some of the more anterior segments are 

 hollow. The amphibian mesoderm is 

 primarily unsegmented and solid. In 

 view of the fact that it later acquires 

 segmentation and hollowness, these ini- 

 tial differences are outweighed by the 

 essential similarity in the relations to the 

 blastoporal region. 



In Reptiles and Birds. In reptiles 

 and birds, endoderm is initiated by a 

 small invagination or infolding at the 

 posterior edge of the early blastoderm 

 (see p. 250). The abortive blastopore thus 

 produced exhibits the usual feature of a 

 blastopore in that, in terms of germ- 

 layers, it is an indifferent region where ectoderm and endoderm merge 





Fig. 216. Surface view of 

 blastoderm of chick after 15 

 hours' incubation. (C) "An- 

 terior crescent," occasioned by 

 an irregular fold of underlying 

 endoderm; (M) region occupied 

 by mesoderm; (O) area opaca 

 whose opacity is caused by 

 adherence of yolk to the blasto- 

 derm; (P) area pellucida — 

 transparent in absence of ad- 

 hering yolk (see Fig. 212); (PS) 

 primitive streak. (X 14.) (After 

 Duval. Courtesy, Neal and 

 Rand: "Chordate Anatomy," 

 Philadelphia, The Blakiston 

 Company.) 



