292 STUDY OF PIG EMBRYOS. 



of its various parts to one another. The hind-brain begins at the spinal cord, 

 Sp.c, and has a very large cavity, the fourth ventricle, Ven.IV. It is separated from 

 the region of the mid-brain by a constriction which is very marked on the dorsal 

 side, Isth. The constriction is known as the isthmus. It is always from the dor- 

 sal side of the isthmus that the fourth nerve takes its origin. It is one of the 

 fixed landmarks of the brain. The mid-brain, M.B, also has a large cavity, and, 

 as a whole, forms the great arch which corresponds to the head-bend of the em- 

 bryo. It passes forward and downward, without any very definite line of demarca- 

 tion at this stage, into the fore-brain, the cavity of which is larger in diameter 

 than that of the mid-brain. The fore-brain is partially subdivided into two regions; 

 the anterior, Pros, is the prosencephalon and gives rise to the lateral outgrowths 

 which form the cerebral hemispheres. Already the deep depression separates this 

 part of the fore-brain on its dorsal side from the posterior part, which is termed 

 the diencephalon. The limits of the diencephalon at this stage are very indistinct; 

 later its boundary against the mid-brain becomes clearly marked by the differentia- 

 tion of the epiphysis and posterior commissure. The spinal cord, Sp.c, forms almost 

 a rightv angle with the axis of the hind-brain. This angle marks and corresponds 

 to the neck-bend of the embryo. On its dorsal side the hind-brain has a thin epen- 

 dymal roof, epen, which, however, toward the isthmus thickens considerably to 

 produce the anlage, Cbl, of the median portion of the cerebellum. On the ventral 

 side the wall of the hind-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 me- 

 dian 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, however, later to be differentiated into the corpora quadri- 

 gemina. 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 essen- 

 tially by a part of the peduncle of the cerebrum. The floor, Dien.fl, of the dien- 

 cephalon is a thin membrane of which the part nearest to the mid-brain will pro- 

 duce the mammary bodies, and the part farther from the mid-brain the tuber 

 cinereum. It has already formed a special outgrowth, Inf, the anlage of the 

 infundibular gland, which extends put 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 infundibular gland in older embryos extends farther on the posterior side of 

 the hypophysis. 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 differentia- 

 tion is the pituitary body, which is really a duplex organ. Below the infundibular 

 gland the wall of the brain shows a thickening, Chi.op, which can be followed 

 through hi the series laterally until it connects with the optic stalk. This thicken- 









