EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 



87 



77). The dorsal convexity is probably abnormal. A robust body-stalk attaches 

 the embryo to the inner wall of the chorion. With the growth of the head- and 

 tail-folds of the embryo, there is 

 an apparent constriction of the 

 yolk-sac where it joins the em- 

 bryo. This will become more 

 marked in later stages and form 

 the yolk-stalk. His's embryo, 

 2.6 mm. long, shows the relative 

 size of yolk-sac and embryo and 

 the yolk-stalk (Fig. 78). The 

 relations of the fetal membranes 

 to the embryo arc much the same 

 as in the chick embryo of five 

 days, save that the allantois of 

 the human embryo is embedded 

 in the body-stalk. The embryo 

 shows a regular convex dorsal 



curvature, there is a marked cephalic bend in the region of the mid-brain and 

 there are three gill clefts. The head is twisted to the right, the tail to the 



Fig. 77. — Human embryo at the commencement 

 of the third week (from His, after Coste). X 15. A, 

 inner or true amnion; A.s., body-stalk; //, heart; V, 

 blood-vessel on yolk-sac; Y.s., yolk-sac. 



Amnion 



Branchial clefts 1-3 



Body-stalk 



Maxillary process 



Mandibular process 

 Heart 



Yolk-sac 

 Fig. 78. — Human embryo 2.6 mm. long showing amnion, yolk-stalk and body-stalk (His). 



