THE 48-HOUR EMBRYO 215 



similar fusion occurs ventrally, but this connection breaks and the heart is 

 freed from the ventral wall of the coelom. 



In the most posterior section through somites (Fig. 133) the gut cavity is 

 absent and the endoderm is simply a thin, flat sheet of cells over the surface 

 of the yolk. As was explained in Chapter 13, this part of the gut, the midgut, 

 becomes a cavity during the folding of the amnion. The mesoderm at this 

 time shows a division into the somite, the intermediate mesoderm, and the 

 lateral mesoderm. The last mentioned is in two layers and encloses the 

 coelom. The somatic and splanchnic layers of the mesoderm of the embryo 

 proper are thick, well-defined layers separate from the ectoderm and endo- 

 derm. Farther out in the extraembryonic area the somatic layer is closely 

 applied to the ectoderm, while the splanchnic layer fuses with the endoderm. 



The 48-hour embryo 



By 48 hours (Fig. 134) the embryo possesses 27 pairs of somites and is 

 7 mm. long. The heart beat and circulation of the blood are well established. 

 The body has twisted so that the left side of the head and neck region is next 

 to the yolk. The right side is uppermost. The ventral side of the posterior 

 body region remains in contact with the yolk. Thus, in this and later stages 

 the head region is viewed from the right side, while the body is seen in 

 dorsal view. The forebrain is brought into close proximity to the heart by a 

 bend of the head and neck. A thin, double layer of somatopleure covers the 

 anterior half of the embryo. The innermost layer of somatopleure is the 

 amnion, and the outer layer is the chorion. 



The boundaries between the embryo proper and the extraembryonic 

 membranes are sharply marked off. The head fold of the amnion together 

 with the lateral folds has progressed to about the twentieth somite. At the 

 posterior end a tail fold develops, which forms a boundary between the 

 posterior parts of the embryo and the extraembryonic area. This leaves a 

 region between the tail fold and about the twentieth somite where the 

 embryonic and extraembryonic areas merge. 



Large vitelline arteries extend from the embryo into the vascular area, 

 where they branch profusely in the mesoderm of the yolk-sac membrane. 

 Blood from this membrane is collected by the vitelline veins and returned 

 to the heart. The complete circulation within an early chick embryo is shown 

 in Figures 101 and 102. 



