THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 79 



joins the embryo. This will become more marked in later stages and form 

 the yolk stalk. 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 al- 

 lantois 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 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 umbihcal 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 (Figs. 84 and 119). It loses its at- 

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



Fig. 84. — Yolk sac and stalk of a 20 mm. human embryo. X 11. 



union with the intestine and is known as Meckel's diverticulum, a structure of 

 clinical importance because it may telescope and cause the occlusion of the intes- 

 tinal lumen. The yolk stalk maj^emain embedded in the umbihcal 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 of the 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 

 in the mid-brain region (cephalic flexure). There are also marked cervical and 

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



