SAGITTAL SECTIONS OF EMBRYO OF 12 MM. 207 



The spinal cord, Sp.c, forms almost a right angle with the axis of the hind-brain. 

 This angle marks and corresponds to the neck-bend of the embryo. On its dor- 

 sal side the hind-brain has a thin ependymal roof, epen, which, however, toward 

 the isthmus thickens considerably to produce the anlage, Cbl, of the median por- 

 tion of the cerebellum. On the ventral side the wall of the fore-brain varies in 

 appearance. Where the section is exactly median, it displays the raphe or floor- 

 plate of the region. Where it is off the median plane, it shows instead the 

 thicker, lateral wall of the medulla oblongata. The walls of the mid-brain on 

 the dorsal side, Q, are almost uniform in thickness and texture. They are, how- 

 ever, later to be differentiated into the corpora quadrigemina. The ventral side 

 of the mid-brain, Ped, is considerably thicker than the dorsal, and forms a 

 strongly marked arch. It is represented in the adult essentially by a part of the 

 peduncle of the cerebrum. The floor, Dien. fl, of the diencephalon is a thin mem- 

 brane of which the part nearest to the mid-brain will produce the mammary 

 bodies, and the part further from the mid-brain the tuber cinereum. It has 

 already formed a special outgrowth, In}, the anlage of the infundibular gland, 

 which extends out from the brain and arches over the end of the hypophysis, Hyp. 

 The hypophysis is an outgrowth from the ectodermal lining of the mouth, Or. 

 Its method of development can be clearly made out at this stage. The infundib- 

 ular gland in older embryos extends further on the posterior side of the hypo- 

 phvsis. Meanwhile the hypophysis loses all connection with the epithelium of 

 the oral cavity, somewhat as does the otocyst with the overlying epidermis 

 which produces it. The hypophysis proper and the infundibular gland undergo 

 their further development in intimate association. The result of their differen- 

 tiation is the pituitary body, which is really a duplex organ. Below the infundib- 

 ular gland the wall of the brain shows a thickening, Chi. op, which can be fol- 

 lowed through in the series laterally until it connects with the optic stalk. This 

 thickening of the brain-wall in later stages furnishes the passage for the fibers of 

 the optic nerve, and is, therefore, the anlage of the optic chiasma. Between the 

 infundibular gland and the optic chiasma extends the post-optic laminae, L.p.o. 

 On the opposite side of the chiasma follows the lamina terminalis, which leads us 

 forward to the wall of the hemispheres, H. Underneath the hind-brain extends 

 the large basilar artery, .4. bos; at its posterior end, A. bas. p, the basilar artery 

 is joined by the two vertebral arteries from the fusion of which it is really pro- 

 duced. Underneath the fore-brain we have the opening of the mouth, Or, from 

 the dorsal side of which springs the elongated evagination of the hypophysis. 

 The oral cavity runs into the pharynx, the floor of which is formed in part by the 

 anlage of the tongue, Ton, and of the epiglottis, Epgl, a rounded eminence very 

 different in shape at this stage from the adult epiglottis. The pharynx can be 





