10 OF 



!N SEGMENTS 



of Cuvier) which open into the dorsal wall of the sinus venosus. The primitive 

 veins are thus paired like the arteries, and like them develop by the enlargement 

 of channels in a network of capillaries. 



The following series of transverse sections from an embryo of this stage shows 

 the more important structures. The approximate plane and level of each section 

 may be seen by referring to Figs. 54 and 55. 



Blastoderm 

 Hind-brain 



Notothori 



MesenchyriL 



Fore-brain 



FIG. 57. Transverse section through the optic stalks and hypophysis of a fifty-hour chick embryo. 





Section through the Fore-brain and Eyes (Fig. 56). The section passes cranial 

 to the optic stalks, consequently the optic vesicles appear unconnected with the fore-brain. 

 The thickened ectoderm is invaginated to form the anlages of the lens vesicles. The thicker 

 wall of the optic vesicles next the lens anlage will give rise to the nervous layer of the retina, the 

 thinner outer wall becomes the pigment layer of the retina. Ventrad in the section are the wall 

 and cavity of the fore-brain, dorsad the hind-brain with its thin dorsal ependymal layer. Be- 

 tween the brain vesicles on either side are sections of the first aortic arches and lateral to the hind- 

 brain are the smaller paired anterior cardinal veins, which convey the blood from the head to 

 the heart. 



Section through the Optic Stalks and Hypophysis (Fig. 57). The section passes 

 just caudal to the lens which does not show. The optic vesicles are connected with the wall of 

 the fore-brain by the optic stalks which later form the path by which the fibers of the optic nerve 

 pass from the retina to the brain. Both the ventral and the descending aorta are seen in section 



