64 TEXT-BOOK OF EMBRYOLOGY. 



as in the frog (Fig. 39), by a breaking through of the two layers — the pro- 

 tentoderm and the yolk entoderm — which separate the invagination cavity 

 from the "completion cavity" in Fig. 42, 2, the two cavities are united to form 

 the archenteron or primitive gut (Fig. 42, 3, 4 and 5). The single-layered germ 

 disk has thus become transformed into a two-layered disk consisting of an outer 

 (upper) layer — the ectoderm — and an inner (lower) layer — the entoderm 

 (protentoderm). 



In Birds the gastrula is formed in a manner quite comparable with its forma- 

 tion in Reptiles. Taking the hen's egg as an example, it will be remembered 

 that the entire segmentation area is confined to the germ disk, and that this con- 

 sists of a superficial layer (roof of segmentation cavity) of small well denned 

 cells (micromeres) beneath which is the cleft-like segmentation cavity, while the 

 floor of this cavity is formed of incompletely segmented yolk (Fig. 29). The 

 beginning of gastrulation is marked by the appearance of a crescentic bar near 

 the posterior margin of the disk. This bar is due to more rapid proliferation 

 of the cells in this region, and in it there appears the crescentic groove or blasto- 



y.c. a.b. arc. 



Fig. 45. — From vertical longitudinal section through two-layered germ disk of nightingale. Hertwig. 

 a.b., anterior lip of blastopore; arc, archenteron; ec, ectoderm; en., entoderm (protentoderm); y.c, 

 yolk cells (merocytes.) 



pore (Fig. 43). Just as described in lower forms, especially Reptiles, the 

 micromeres invaginate or fold under at this point and grow forward as the 

 protentoderm, and roof in the new cavity formed by the invagination (Fig. 44). 

 The single-layered germ disk is thus transformed into a two-layered disk con- 

 sisting of an outer (upper) layer — the ectoderm — and an inner (lower) layer — 

 the entoderm (protentoderm). The protentoderm in a sense replaces the 

 original layer of yolk cells in the area where the invagination occurs; the original 

 outer layer (micromeres) becomes the ectoderm, except that portion which is 

 invaginated to form the protentoderm (Fig. 45). This process is comparable 

 with the disappearance of the yolk entoderm in Reptiles (Fig. 42). At the same 

 time the segmentation cavity is obliterated and the new cavity — invagination 

 cavity — which is in communication with the exterior, appears beneath the 

 protentoderm. (Compare Figs. 42 and 45.) 



Under the central portion of the germ disk the yolk becomes liquefied, 

 while at the margin of the disk it continues to segment and give rise to large 

 nucleated cells — the yolk entoderm. This is known as the area of supplemental 



