54 



CHORDATE ANATOMY 



permanently endoderm (Fig. 4.0C, EN). That is, the two materials 

 together and in no way delimited from one another constitute the deeper 

 layer of the overgrowing fold. Later this layer virtually splits (the 

 process called delamination) to form two layers, an inner one abutting on 

 the archenteric cavity and an outer one which is then recognizable as a 

 definite mesoderm (Fig. 44B). This layer, although now distinct from 

 the endoderm which parallels it, for a time retains continuity with its 

 source, the proliferation zone about the blastopore (Fig. 44-4). Initiated 

 in this way, the mesoderm extends into the lateral and anterior regions 

 of the embryo partly by growth within itself, partly by continued con- 

 tributions from the blastoporal growth zone and possibly augmented 

 by the detachment of cells from neighboring surfaces of the endoderm. 



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 hollow- 

 ness these initial 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 page 48). 

 The abortive blastopore thus produced 

 exhibits the usual feature of a blastopore in 

 that, in terms of germ layers, it is an in- 

 different region where ectoderm and 

 endoderm merge together without sharp 

 demarcation (Fig. 41). Following gastru- 

 lation the blastodermal lavers continue to 







■t~-o 



^A-V 



V 



, iit2c p 



-y^_^ 



*■ ■J' 





Fig 45 — Surface view of 

 blastoderm of chick after 15 hours 

 incubation. C, "anterior cres- 

 cent," occasioned by an irregular 

 fold of underlying endoderm; M, 



region occupied by mesoderm; spread rapidly over the surface of the yolk. 



the blastoderm; P, area pellucida 

 — transparent in absence of ad- 

 hering yolk (see Fig. 41); PS, 

 primitive streak. X 14. (After 

 Duval, "Atlas d'Embryologie.") 



O, area opaca whose opacity is , • 1 1 • 1 



caused by adherence of yolk to In SO domg, the growth posteriorly causes 



the somewhat thickened region of the blas- 

 toporal rim to become drawn out into a long 

 streak, the primitive streak, lying in the 

 median line of the blastoderm (Fig. 45). 

 Along the whole extent of this modified blastoporal region the ectoderm 

 and endoderm merge without sharp demarcation just as they did in the 

 earlier blastoporal walls (Fig. 46). 



This primitive streak is the primary seat of mesoderm formation. 

 Rapid proliferation of cells within the substance of the thickened streak 

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



