212 EMBRYOLOGY 



area vasculosa, since it gives rise to the profusely branched blood vessels. 

 Within the next 12 hours the blood vessels of the area vasculosa will join 

 with others formed in the embryo and a complete circulation will be 

 established. 



Figures 125 through 128 show typical sections through the region of the 

 forebrain, through the intestinal portal, through a pair of somites and through 

 the primitive streak. 



Development during the second day 



Figure 129 shows a stained preparation of an embryo with 14 somites and 

 corresponds to about 34 hours of incubation. The length of the embryo is 

 about 4.5 millimeters. By this time the neural tube is closed throughout most 

 of its length and the anterior region has formed three clearly defined divi- 

 sions — prosencephalon, with optic vesicles; mesencephalon; and rhomben- 

 cephalon. The auditory pits are present opposite the latter. The heart has 

 formed, and the two vitelline veins, which have fused with the endocardial 

 rudiments, are prominent. The heart contracts at this time, but the blood has 

 not yet begun to circulate, since all the connections are not completed. The 

 heart beat is at first irregular but soon becomes steady. In a few hours the 

 blood will begin to circulate. Posteriorly the primitive streak is still visible. 



Figures 130 to 133 are cross sections through four regions of a 33-hour 

 chick embryo. At the level of the optic vesicles the head is thrust forward 

 over the ectoderm and the space between it and the head is called the sub- 

 cephalic pocket. The anterior tip of the foregut shows in this section. Under 

 the head there is as yet no mesoderm between the ectoderm and the endoderm. 

 Lateral to the head, however, layers of splanchnic and somatic mesoderm 

 can be seen enclosing a large coelom, the extraembryonic coelom. The 

 somatopleure on each side of the head will fold over the dorsal surface of 

 the head to form a chorion and an amnion. The splanchnopleure will form 

 the yolk sac. 



The foregut widens posteriorly (Fig. 131), and the roots of the dorsal 

 and ventral aorta form in the mesenchyme of the head region. The section 

 is at the level of the mesencephalon. More posteriorly at the heart level 

 (Fig. 132) the epimyocardium is shown compressing the floor of the foregut 

 into a V shape. At this point the mesoderm fuses to form the dorsal 

 mesocardium, which anchors the heart to the dorsal wall of the coelom. A 



