34 



BIOLOGY OF THE LABORATORY MOUSE 



— Ectoplacental cone 

 Reichert's membrane 



Ectoplacental cavity 



Extra-embryonic 

 coelom 

 Yolk-sac 



Yolk cavity 



Amniotic cavity 



Embryonic ectoderm 

 Primitive streak 



Allantois 



Notochord 



Ectoplacenta 



Chorion 



Yolk-sac 

 Amnion 

 Amniotic cavity 

 Extra-embryonic'coelom 



Neural groove Intra-embryonic coelom 



Fig. 19. — Frontal sections (X75) showing development of the yolk-sac. A. 

 7 days 6 hours. B. 7 days 10 hours. C. 8 days 10 hours, 9 somites, through 8th 

 somite. Reichert's membrane omitted except in \. 



