EMIiRYO OF TWENTY-SEVEN SEGMENTS 



61 



TRANSVERSE SECTIONS 



Section through the Fore-brain and Eyes (Fig. 58). The section passes in front of 

 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 re- 

 tina; the thinner outer wall becomes the pigment layer of the retina. Ventrad in the sec- 

 tion are the wall and cavity of the fore-brain, dorsad the hind-brain with its thin, dorsal 

 rpendymal layer. Between 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. The splanchnopleure of the blastoderm is 

 characterized in this and subsequent sections by the presence of blood vessels in its meso- 

 dermal layer. 



Mesoderm 



Mesoderm of bitlbus 

 Endothelium of bulbus 



FIG. 60. Transverse section through the otic vesicles and second aortic arches of a fifty-hour 



chick embryo. X 50. 



Section through the Optic Stalks and Hypophysis (Fig. 59). The section passes 

 just caudal to the lens. 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 

 about the cephalic end of the pharynx. Between the ventral wall of the fore-brain 

 and the pharynx is an invagtnation of the ectoderm, Rathke's pouch (anterior lobe of the 

 hypophysis). 



Section through the Otocysts and Second Aortic Arch (Fig. 60). The otic vesicles 

 are sectioned caudal to their apertures and appear as closed sacs, lateral to the wall of the 

 hind-brain. The cavity of the pharynx is somewhat triangular and its dorsal wall is thin. 



