TIIK ANATOMY OF A 4.2 MM. HUMAN KMI1KYO 8l 



sac where it joins the embryo. This, however, is a deception. Both 

 embryo and yolk sac enlarge, whereas the region of union lags and later 

 becomes the slender yolk stalk (Fig. 84). His' embryo, 2.6 mm. long, 

 shows the relative size of yolk sac and embryo and the yolk stalk (Fig. 

 83). The relations of the fetal membranes to the embryo are 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 branchial grooves. The head is 

 twisted to the left, the tail to the right. 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. 



In later stages, with the development of the umbilical cord, the 

 yolk stalk becomes a slender thread extending from the dividing line 



FIG. 84. Yolk sac and stalk of a 20 mm. human embryo. X 1 1. 



between the fore- and hind-gut to the yolk sac, or umbilical vesicle (Figs. 

 84 and 119). It loses its attachment to the gut in 7 mm. embryos. A 

 blind pocket may persist at its point of union with the intestine; this is 

 known as Meckel's diverticulum, a structure of clinical importance because 

 it sometimes telescopes and causes the occlusion 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. The yolk sac may be persistent at birth. 



THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 



This embryo, studied and described by His, is regarded by Keibel 

 as not quite normal. Viewed from the left side (Fig. 85), 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 body stalk. There is an indication of 

 the primitive segments along the dorso-lateral line of the trunk. The 

 head is bent ventrad almost at right angles, forming in the mid-brain 

 region the cephalic flexure. There are also marked cervical and caudal 

 flexures, the trunk ending in a short, blunt tail. The heart is large and 



