76 



COMPARATIVE ANATOMY 





-o 



The mesoderm of Amphioxus is segmented at the time of its detach- 

 ment from the primary gastrular layers and some of the more anterior 

 segments are hollow. The amphibian mesoderm is primarily unseg- 

 mented and solid. In view of the fact that it later acquires segmentation 



and hollowness, these initial differences are 

 outweighed by the essential similarity in 

 the relations to the blastoporal region. 



In Reptiles and Birds. In reptiles and 

 birds the formation of an endoderm is 

 initiated by a small invagination or infolding 

 at the posterior edge of the early blastoderm 

 (see page 69). 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 together without sharp 

 of demarcation (Fig. 45). Following gastrula- 

 tion the blastodermal layers continue to 







-PS 





Fig. 49. — Surface view 

 blastoderm of Chick after 15 hours 

 incubation. C, "anterior cres- 

 cent," occasioned by an irregular Spread rapidly over the surface of the yolk. 

 fold of underlying endoderm; M. j^ go doing, the growth posteriorly causes 



region occupied by mesoderm; , . 



O, area opaca whose opacity is the somewhat thickened region of the 

 caused by adherence of yolk to blastoporal rim to become drawn out into a 



the blastoderm; P, area pellucida 



long Streak, the "primitive streak," lying 

 in the median line of the blastoderm (Fig. 

 49). Along the whole extent of this modi- 

 fied blastoporal region the ectoderm and 



endoderm merge without sharp demarcation just as they did in the 



earlier blastoporal wall (Fig. 50). 



This primitive streak is the primary seat of mesoderm formation. 



Rapid cell proliferation within the substance of the thickened "streak" 



— transparent in absence of adher- 

 ing yolk (see Fig. 45); PS, 

 primitive streak. X14. (After 

 Duval, Atlas d'Embryologie.) 



y--^:^~^t 



Fig. 50. — Section transverse to the primitive streak of a chick embryo of about 15 

 hours incubation. The section is taken near the middle of the length of the streak. EC, 

 ectoderm; EN, endoderm; MES, mesoderm; PG, primitive groove of primitive streak; 

 Y, yolk at inner margin of area opaca. Xioo. (After Duval, Atlas d'Embryologie.) 



gives rise to masses of cells which move out into the space between ecto- 

 derm and endoderm (Fig. 50, MES). These masses of cells increase by 

 continued contribution from the "streak" and by growth within them- 

 selves and soon become arranged in a layer which rapidly grows laterally 



