8 4 



THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 



during its growth, may cause scars and splitting of eyelids or lips. Such amniotic threads 

 may even amputate a limb or cause the bifurcation of a digit producing a type of polydac- 

 tylism. 



The Allantois. — The allantois appears very early in the human embryo be- 



A 



Yolk-sac 



Amnion 

 Neural groove 



Neurenteric canal 

 Primitive streak 

 Body-stalk 



Villi of chorion 



Amnion 

 Embryonic 

 disc 



Chorion 

 Mesoderm 



Rudiment 

 of heart 



Mesoderm 



Body-stalk 



Primitive 



streak 



Allantois 

 Yolk-sac 



Entoderm 



- Blood-vessel 



Fig. 73. — Views of a human embryo 1.54 mm. long. A, dorsal surface; B, median sagittal section 



(Graf Spee). 



