222 



EMBRYOLOGY 



EXTRAEMBRYONIC 

 COELOM 



VITELLINE ARTERY 



SOMITE 

 NEPHROTOME 

 LATERAL MESODERM 



DORSAL AORTA 



Fig. 143. The vitelline arteries. 



ECTODERM 



Fig. 144. The tail bud and the formation of the hindgut. 



The last figure (Fig. 144) shows the tail bud and the formation of the 

 hindgut. A posterior fold and lateral folds close off the endoderm into a 

 pocket. The tail bud represents the last vestiges of the primitive streak and 

 is a mass of undifferentiated cells. The mesoderm, endoderm, and ectoderm 

 fuse within the tail bud. 



The 72-hour chick embryo 



From 48 to 72 hours development is characterized by a progressive dif- 

 ferentiation of the structures present at 48 hours. Some new structures also 

 appear, and the number of somites increases to 36 pairs. The embryo is larger 

 but because of its curvature the over-all length is still about 7 mm. It weighs 

 about 3 mg. The vascular area continues to spread in an ever widening circle 

 over the surface of the yolk. The amnion completely covers the embryo, and 

 a small allantois has grown out of the hindgut. As in the 48-hour chick 

 embryo the head and neck lie on their right sides and the trunk is seen from 

 a dorsal view (Fig. 145). 



The most anterior region of the brain, the telencephalon, is close to the 

 ventricle of the heart. Paired cerebral hemispheres begin to differentiate 

 from the telencephalon. The next division of the brain is the diencephalon, 

 with the prominent eyes on either side. As we continue around the curve of 



