224 FREDERICK TILNEY AND LUTHER F. WARREN 



cited. This is of particular significance because this list in- 

 cludes the names of those who have given the most extensive 

 attention to the histological character of the epiphysis in 

 mammals. Milhalkovicz' 274 conception of the histology of the 

 pineal body seems hardly tenable, for it requires little study 

 covering a number of different mammalian forms to become con- 

 vinced that the cellular elements entering into the epiphysis 

 have nothing in common with the ependymal cells. Even 

 though it may be admitted that a certain number of the cellular 

 constituents of the epiphysis are ependymal in type, it cannot, 

 in the light of our present knowledge, be held that the organ is 

 made up exclusively of this type of cells. 



On the other hand, it is not possible to acceed to the conten- 

 tion of those who uphold the idea that the epiphysis is similar 

 to lymphatic glands. Not only does the character of the chief 

 cellular elements in the pineal body of mammals make this 

 position seem untenable, but even more does the arrangement 

 of these cells point away from the supposition that this is in 

 any sense lymphoid in character. Few cells in the body are 

 more conspicuous for their histological character than the chief 

 or parenchymatous elements of the mammalian epiphysis. 

 The large and centrally placed nucleus, the extensive and glan- 

 dular cytoplasm mark these cells so definitely that they may be 

 recognized without any difficulty even in those instances when 

 they become ectopic because of such migration as not infre- 

 quently results from tumor formation in the pineal body. 



Our own work in this regard is illustrated in the figures which 

 show the character of the pineal gland cells in Macropus grayi, 

 Zalophus, Camelus dromedarius, Capra hylocrius, Lepus, Simia 

 satyrus, and in man. Furthermore, our observations in the 

 ontogenesis of the epiphysis hi Felis domestica and in man, 

 illustrations of which are given in figures 91 and 92, show that 

 in the early stages of differentiation the nuclei of the ependymal 

 cells are so large and the cytoplasm so scanty that they give 

 the impression of lymphoid tissue, but in the later stages the 

 cytoplasm increases so considerably in amount that it is no 

 longer possible to conceive of these cells as lymphoid in char- 





