276 



EMBRYOLOGY 



coelom. The two heart rudiments have fused to form a single tube of endocar- 

 dium surrounded by a thicker wall of myocardium. At about this time, or per- 

 haps even earlier, the heart begins to beat. Some blood cells formed by the 

 blood islands of the yolk sac are present, and circulation through the yolk sac 

 and the body stalk is complete. 



Up until this time the embryo has had no nutrition through the placenta 

 and has had to rely on diffusion of products from the uterus (uterine milk) 

 and on products of disintegration of cells of the wall of the uterus. These 

 cells break down during implantation to form a debris which is used as food 

 by the growing embryo. With the establishment of a complete circulation at 

 the third week, the embryo derives its food through the placenta, as outlined 

 in Figure 168. 



Many substances pass through the placental barrier. In general, only the 

 smaller molecules penetrate the three layers of cells that separate the fetal 

 and maternal blood. However, some antibodies of large molecular size do 

 pass from mother to fetus and confer immunity on the latter. Although the 

 acquisition of some antibodies is beneficial for the newborn child, other in- 

 jurious large molecules also can get through the placenta — for example, the 

 anti-Rh factor, which produces erythroblastosis. Furthermore, if the mother 



ORAL CAVITY 



BULBUS 



YOLK SAC 



: %-OTOCYST 



ARM BUD 



BODY REGION 



LEG BUD 



BODY STALK 



Fig. 185. A human embryo about 26 days old. (From Julius Kollman, 

 Handatlas der Entwickhtngsgeschichte des Memchen. Verlag von Gustav Fischer, 

 Jena.) 



