4 6o SCIENCE PROGRESS 



Aves. — The pituitary of birds in its general features closely 

 resembles that of Reptiles. 



The pars neuralis is hollow and is sometimes convoluted 

 (Herring, 8). The pars buccalis is of considerable size, and 

 contains no lumen, but Herring states that the position of the 

 cleft is represented by a small amount of connective tissue 

 lying between the pars distalis and the strands of the pars 

 infundibularis. 



The pars infundibularis consists of strands of epithelial 

 cells closely investing the pars neuralis. The pars distalis con- 

 sists of cell columns showing only slight differentiation in 

 staining capacity. 



The pars tuberalis, according to Tilney (22), forms in the 

 common fowl a ring-shaped structure around the base of the 

 pituitary stalk. It arises as in Reptiles from lateral buds which 

 grow up so as to come into contact with the brain and then 

 increase in size to such an extent that the two originally 

 separate lateral structures meet and unite both anterior and 

 posterior to the pituitary stalk and so displace the pars distalis 

 slightly, ventralwards, removing it from contact with the 

 brain floor. 



The pars buccalis of birds arises as a thin-walled ectoder- 

 mal invagination which closes to form a hollow pouch. The 

 development of the pars tuberalis has already been outlined. 



Mammalia. — The hypophysis of the Metatheria closely 

 resembles that of the Eutheria, so that it is possible to deal 

 with the group as a whole. 



In the general relations of the parts, the hypophysis of 

 Mammals closely resembles that of birds and reptiles. The 

 pars neuralis is large and well-developed, and, in most cases, 

 is solid. In the cat, however, it retains a lumen. Herring (7) 

 states that the pars neuralis of the Mammalian pituitary con- 

 tains no true nervous elements, but consists entirely of ependy- 

 mal and neuroglial cells and fibres. The relations of the pars 

 neuralis and pars infundibularis are extremely intimate in 

 Mammals (fig. 4) ; the cells of the pars infundibularis pene- 

 trate into the tissues of the pars neuralis (P.N.) so that the 

 so-called posterior lobe (P.L.) is a complex structure. 



The pars buccalis retains the original lumen of Rathke's 

 pouch as a cleft (L), generally cup-shaped, separating the pars 

 infundibularis from the pars distalis. The pars distalis (P.D.) 



