X.] 



THE HUMAN EMBRYO. 



339 



The embryo is still but incompletely folded off from 

 the yolk-sac below. 



In a still older stage the cranial flexure becomes 

 still more pronounced, placing the mid-brain at the end 

 of the long axis of the body. The body also begins to 

 be ventrally curved (Fig. 110 C). 



Externally human embryos at this age are charac- 

 terized by the small size of the anterior end of the 

 head. 



The flexure goes on gradually increasing, and in the 

 third week of pregnancy in embryos of about 4 mm. the 

 limbs make their appearance. 



The embryo at this stage (Fig. 112), which is about 



FIG. 112. 



Two VIEWS OP A HUMAN EMBRYO OF BETWEEN THE THIRD 

 AND FOURTH WEEK. 



A. Side view. (From Kolliker ; after Allen Thomson.) a. 

 amnion ; b. umbilical vesicle ; c. mandibular arch ; e. hyoid 

 arch; /. commencing anterior limb; g. primitive auditory 

 vesicle ; h. eye ; i. heart. 



B. Dorsal view to shew the attachment of the dilated allantoic 

 stalk to the chorion. (From a sketch by Allen Thomson.) 

 am. amnion ; all. allantois ; ys. yolk-sac. 



222 



