THE DIENCEPHALIC FLOOR j 271 
observed in this region in the cat embryo of 25 mm. length al- 
most exactly duplicate those in the brain of the 20 mm. Mustelus, 
but as in the case of the chick, the tendency of this region to 
give rise to large inferior lobes diminishes as growth progresses 
and in the fetal stages the only evidence of this tendency is to 
be found in the large post-chiasmatic eminence and the post- . 
chiasmatic recess. Significant in this connection also is the 
relation which the pituitary gland bears to this part of the 
infundibular region. As the writer (8) has already shown, the 
post-chiasmatic eminence in the cat and chick comes to be in- 
vested by a secondary outgrowth from Rathke’s pocket which 
ultimately forms the pars tuberalis of the gland. In the selachian 
no such investment of the inferior lobes takes place, but the 
tuberal portion of the gland by a process of development similar 
to that in the bird and mammal grows forward and occupies 
a juxta-neural position in contact with the small portion of the 
infundibular region which forms the median post-chiasmatic 
groove. 
The caudal portion of the infundibular region including the 
apex is involved in the formation of the neural portion of the 
hypophysis. In its inception the developmental process of this 
part in all the forms studied presents a marked similarity. The 
area about the apex of the infundibular region undergoes a sub- 
- division so that two evaginations are formed, one dorsal and the 
other ventral. In the cat this subdivision gives the dorsal 
evagination the greater size from its beginning, while in the bird 
and the dog-fish the ventral evagination is the larger. In all 
instances the ventral evagination proceeds to the formation of 
the neural portion of the hypophysis, the infundibular process. 
In the salachian this process is but little pedunculated. A slight 
constriction does, however, occur and justifies the term infundib- 
ular process for the structure which is connected with the floor 
of the interbrain by a short infundibular stem. The characteris- 
tic features of this infundibular process are the development of 
two morphologically different surfaces one which is ventral, 
thin and non-convoluted coming in contact with the pituitary 
gland, the other which is thin-walled and dorsal in position, but 
