EMBRYO OF TWENTY-SEVEN SEGMENTS 



59 



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

 may be ascertained by referring to Figs. 56 and 57. 



Transverse Sections 



Section through the Fore-brain and Eyes (Fig. 58).— 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. 



Blastoderm 

 Hind-bra I i—M^ 



Amnion 

 Chorion 



Anl. cardinal vein 



Notochord^ 

 Descending aoria 

 Ant. cardinal vein. 

 Ventral aorta' 



Optic vesicle. 

 Mesencli \<mc. 



— Rallike's pocket 



Fig. 59. — Transverse section through the optic stalks and hypophysis of a fifty-hour chick embryo. 



X50. 



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 carditial veins, which convey the blood from the 

 head to the heart. 



Section through the Optic Stalks and Hypophysis (Fig. 59). — 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 about the cephalad end of the pharynx. Between the ventral wall of the fore-brain 

 and the pharynx is an invagination of the ectoderm, Rathke's pocket (anterior lobe of 

 hypophysis). 



