136 Herring 



opitlielium of tlie pars intermedia is shown in %. 6, which is a drawing 

 from a sagittal section through part of both lobes of the pituitary of the 

 cat. In this specimen and in the one from which fig. 7 is taken, the blood- 

 channels are collapsed and their position is indicated by endothelial cells. 

 The cells are arranged in solid columns, between which run thin-walled 

 blood-channels. The columns show no central lumen, nor is there any 

 colloid met with either in the cells or between them. Where colloid is 

 present it lies among the clearer cells of the pars intermedia, and never in 

 relation to the characteristic granular cells of the anterior lobe. 



The cells of the anterior lobe and of the pars intermedia are derived 

 from the same origin and become differentiated during fcetal life. In the 

 kitten the cells of the anterior lobe do not show such marked differences in 

 size and the possession of granules as they do in the adult. The clear cell 

 and the granular cell are recognisable, and there are transitional forms. 

 While it is almost impossible as yet to settle the exact nature of these cells, 

 I am inclined to believe that all the varieties represent varying stages of 

 functional activity of one and the same kind of cell, and that the deeply- 

 staining material is the product of the cell destined to be poured as an 

 internal secretion into the blood-vessels. 



The blood-vessels of the anterior lobe are extremely numerous and wide. 

 When injected with carmine gelatine from the carotids they are seen to 

 form wide channels resembling to some extent the sinusoids of the liver. 

 The endothelial cells are closely applied to the epithelial cells without 

 intervening connective tissue cells. In this respect also they resemble the 

 sinusoids of Minot. There is, however, no evidence of any intracellular 

 canalisation of the epithelial cells, such as is found in the liver (17). A fine 

 reticulum of connective tissue is present in most places, resembling the 

 " Gitterfasern " of the liver lobules. Whether lymphatics exist or not is 

 doubtful ; the sinusoidal character of the blood-vessels and the closely fitting 

 endothelial cells render their presence unlikely in many parts of the anterior 

 lobe. In certain situations near the cleft and pars intermedia true capillaries 

 and connective tissue are found, and lymphatic vessels appear to exist in 

 these situations. 



The anterior lobe in tlie cat is usually separated from the cleft by a 

 single layer of flattened cells, which are larger than endothelial cells, and 

 are continuous at the anterior and posterior ends of the cleft with the cells 

 of the epithelial reflection (fig. 6). 



In the dog the anterior lobe is permeated by extraordinarily large, thin- 

 walled blood sinuses. Lothringer (22) compared the structure of the 

 anterior lobe in this animal to cavernous tissue. In the monkey, too, the 

 blood-vessels are in the form of wide, thin- walled sinuses running more or 

 less parallel to one another in an antero-posterior direction. 



The changes in structure of the anterior lobe which have been alleged 

 to follow thja-oidectomy in the rabbit require further investigation. The 

 normal variation in structure and arrangement of the cells varies within 



