REPTILES AND BIRDS 



377 



be rolled about or be subjected to blows from without. The outer wall 

 of the amniotic fold is continuous with a fold of the somatopleure which 

 grows down around the yolk sac. As the splanchnopleure also extends 

 down around the yolk sac a space is formed between the somatopleure 

 and splanchnopleure, the wall of which is mesoderm and which is known 

 as the extra-emhryonic coelom (Fig. 256 D). 



Nerve corcf 

 Nofochord 



Nerve corcf 



Nofochord 



Coelom 



f^ o o Q ^ o o o o o. .o. 0.0,0,0 o o 



o * o ' o • o o , o ■ 

 o o-o-°' o 00 • 



0.0 o 



Coe/om 



B 



Amniotic f„^u^ 

 fold . Embryo 



o ' o o o 0-0 

 o ■ 0.0.0 



/4 mniofic 

 fold 



Amnionic 

 coiv/'fy 



£x frof-embryonic 

 coelom 



Shell 



'hi Cavity of 

 ■:^^ al Ian to IS 



Exfra-ernbryonic 



coe/om Amniotic 



cavity 



A I loin to ic 



s,., stalk 

 : oS-.. 



oo:°\. 



Allcrntois 



Fig. 250. — Diagrams of the development of a bird's egg. A, cross section of an 

 amphibian embryo for comparison with B, which is a cross section of an avian embryo 

 at an early stage. C, D, and E, stages in the development of amnion and allanfois in the 

 bird, shown in longitudinal section. Ectoderm is shown in C, D, and E by a solid line, 

 entoderm by dashes, mesoderm in mass by crosslines, and somatic mesoderm and splanchnic 

 mesoderm by dots. 



409. Allantois. — Since the embryo needs oxygen, a means must be 

 provided for respiration. This is afforded by the allantois, which is an 

 outpocketing of the enteron posterior to its connection with the yolk 

 sac, the wall of which consists therefore of entoderm and splanchnic 

 mesoderm. This outpocketing projects into the extra-embryonic 

 coelom (Fig. 256 D) and as it develops it expands mushroomlike against 

 the outer wall of that cavity (Fig. 256 E). The amnion and allantois 

 together form what has often been termed a chorion, which is spread 



