88 



THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 



left. At the side of the oral sinus are two large processes; the dorsal of these 

 is the maxillary, the ventral the mandibular process. The heart is large and 

 flexed in much the same way as the heart of the fifty-hour chick embryo. 



Fig. 79. — Yolk-sac and-stalk of a 20 mm. human embryo. X n. 



AI id-brain 



In later stages, with the development of the umbilical cord, the yolk-stalk 

 becomes a slender thread extending from the dividing line between the fore- and 

 hind-guts to the yolk-sac or umbilical vesicle (Fig. 114). It loses its attach- 

 ment to the gut in 7 mm. embryos. A blind pocket may persist at its point of 



union with the intestine and is known 

 as Meckel's diverticulum, a struc- 

 ture of clinical importance because 

 it may telescope and cause the oc- 

 clusion of the intestinal lumen. The 

 yolk-stalk may remain embedded in 

 the umbilical cord and extend some 

 distance to the yolk-sac which is 

 found between the amnion and 

 chorion (Fig. 79). The yolk-sac 

 may be persistent at birth. 



Hind-brain 



i nditorv vesicle 



Branchial 

 arches 



Amnion (cut) 



Body-stalk 



THE ANATOMY OF A 4.2 MM. HUMAN 

 EMBRYO 



Fig. 80. — Left side of a human embryo of 4.2 mm. 

 (His). 



This embryo, studied and de- 

 scribed by His, is probably not 

 quite normal. It shows a concave 

 dorsal flexure which Keibel regards as due to distortion. Viewed from the 

 left side (Fig. 80), with the amnion cut away close to its line of attachment, 

 there may be seen the yolk-stalk, and a portion of the yolk-sac and of the 



