246 MR HAROLD AXEL HAIG ON 



packed cells, which curves round the ventral aspect of the posterior lobe and appears 

 to end abruptly in a broad club-shaped extremity at the posterior and ventral margin 

 of that lobe. The dorsal portion, however, passes over the upper surface of the pars 

 anterior and seems to fuse with the anterior and dorsal margin of that lobe. No 

 remnant of Rathke's pouch is to be seen in the fully formed pituitary of the Seal, 

 whereas in the rabbit, cat, and man the pouch persists as a distinct cleft between the 

 pars anterior and the pars intermedia. 



In the present instance the roof of Eathke's pouch is lined by a columnar 

 epithelium, which at the anterior extremity merges into the cell-columns which later 

 on fill up the pouch. Upon the dorsal aspect of the posterior lobe no epithelial 

 investment is to be seen, whilst the floor of Rathke's pouch is formed by the super- 

 ficial cells of the pars anterior, which are arranged in the form of an epithelium. 



The very close union maintained between the buccal and cerebral portions of the 

 pituitary from the earliest stages is commented upon by Herring # and emphasised 

 as having a direct bearing upon the functional importance of the gland. In the 

 Seal, at the stage of development now under discussion, the union between 

 the two portions is very intimate, but becomes even more marked as develop- 

 ment proceeds. 



Compared with the hypophysis of a three-months human fetus, that of this Seal 

 is a good deal in advance. In the human foetus at three months the connecting 

 strand passing between the pars anterior and the buccal epithelium is still present, 

 whilst the posterior lobe is only represented by a very narrow diverticulum from the 

 floor of the 3rd ventricle. 



Again, sections taken sagittally through the pituitary of the rabbit three days 

 before birth show that the size of the posterior lobe is small as compared with the 

 anterior, and the cavity in the infundibulum is represented by a mere cleft lined by 

 ependyma. During earlier phases in this animal (twelfth and fourteenth days) the 

 relation between the size of the lobes is such that the posterior lobe is proportion- 

 ately larger than during later stages. In the Seal, at the stage under discussion, the 

 posterior lobe is at least one-half the size of the pars anterior ; but this relation does 

 not hold in the full-grown animal, where the anterior lobe is five or six times as large 

 as the pars nervosa. 



More detailed histological examination of the pituitary of the present fetus 

 shows certain characters which differ from those observed in similar anatomical 

 regions during later stages ; in the first place, the blood-channels in the pars anterior 

 still retain their endothelial lining, a feature which disappears during development 

 to a large extent, although in some parts of the full-grown anterior lobe endothelial 

 cells are to be seen forming at least a partial lining to the blood-vessels. The cell- 

 columns of the anterior lobe are composed of closely packed polyhedral cells, which, 

 when stained with an acid stain such as eosin, may be differentiated into two 



* Op. cit. 



