384 



METAZOAN PHYLA 



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-embryonic coelom (Fig. 270 D). 



^Nerve corcf 

 ■■Nofochorct 



Nerve cord 



No to chord 



Coelom 



o . o o y ; 



.0.0 o o 

 'o ■ O o o . o 



o ■ 0.0.0 



Amniotic 

 fold 



A mniofic 

 fold 



A mniofic 

 coiv/fy 



Extra-embryonic 

 coelom 



Shell 



Extra-embryonic 

 coelom Amniotic 



cavity 



Yolk 

 stalk 



O o' *> 



^' o. o o o 



■ 00" 



Allahtois 



Fig. 270. — ^Diagrams of the development of a bird's egg. A, cross section of an 

 amphibian embryo for comparison with 5, which is a cross section of an avian embryo 

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

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

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

 mesoderm by dots. 



411. 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. 270 D) and as it develops it expands mushroomlike against 

 the outer wall of that cavity (Fig. 270 E). The somatopleure and allan- 

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



