THE SYSTEMATIC ANATOMY OF A FCETAL SEA-LEOPARD. 



245 



(ii.) An intermediate cleft lying dorsal to the pars anterior, which is the remaining 

 evidence of Rathke's pouch ; this cleft is towards its lateral aspects (not 

 shown in the figure) partially filled by proliferating columns of epithelial 

 cells, some of which are to be seen in the mesial section at the anterior 

 extremity of the cleft. 



(iii.) The pars nervosa, or posterior lobe, lying dorsal to the cleft, and connected 

 with the floor of the 3rd ventricle by the infundibulum ; part of the cavity 

 of the ventricle is seen extending into the infundibulum, but this feature 

 disappears during later stages to a large extent. 



Fig. 3. — A vertical section through the base of the skull to show the hypophysis cerebri in situ, lying 

 in the sella turcica of the sphenoid bone. (Semi-diagrarnmatic.) 



a. Epithelium of the roof of the nasal cavity. 



b. Developing basisphenoid bone ; the ' ' irruption "-stage of ossification in cartilage is represented. 



c. Large anterior lobe of hypophysis showing cell-columns and intervening blood-vessels ; the 



dark masses are cells staining more deeply with eosin. 



d. Infundibulum, showing communication with the 3rd ventricle of brain. 



e. Posterior lobe (pars nervosa) of hypophysis. 



/. Cleft between the anterior and posterior lobes ; at the anterior end of this is seen a mass of 

 cells, derived by an infolding from the anterior lobe, which will ultimately give rise to the 

 pars intermedia. 



g. Cavernous sinus. 



h. Dorsum sellae of sella turcica. 



In connection with the pars intermedia, it appears that the columns of cells found 

 partially filling the above-mentioned cleft are derived by a proliferation of cells at 

 the upper and anterior angle of the pars anterior. Herring # describes (in the cat) 

 the pars intermedia as an epithelial investment of the posterior lobe, but in the Seal, 

 at least during later stages, the intermediate mass is a well-defined strip of closely 



* Joum. of Exper. Physiol., 1909. 



