THE PINEAL BODY 189 



ually extends to its distal extremity. Many of these diver- 

 ticula remain in connection with the third ventricle, but as they 

 elongate toward the tip of the pineal body many of the diver- 

 ticula lose this connection and finally appear as blind acini or 

 cell cords. In this way the original more or less indifferent cell 

 area of the primitive anlage is invaded by cells from the diver- 

 ticula above described. Simultaneous with the invasion of 

 these diverticula, blood vessels are seen to make their way into 

 the tissue between the acini and cell cords. This vascular 

 invasion seems to take place from the periphery going to the 

 center, but it is possible that independent blood spaces are 

 formed which, by concresence, subsequently form a vascular 

 network, the latter coming into relation with the blood vessels 

 surrounding the pineal body. These characters of the onto- 

 genesis of the pineal body in the cat are shown in figure 91. 



The process just described in the histogenesis of the cat is 

 much better illustrated in the development of the human fetus. 

 In man, the process of diverticular invasion into the original 

 cellular mass of the primitive anlage is well shown in figure 92, 

 representing the condition in a human fetus of six months. Here 

 it will be noted that the invasion begins at the base of the epi- 

 physis and manifests itself in the thick strand of darkly staining 

 cells extending out and into a mass of undifferentiated tissue. 

 At term the invasion has extended completely through the epi- 

 physis and the deeply staining strands of cells are now arranged 

 in convoluted cords or take the form of apparent acini. In the 

 meshes between these cords capillaries appear to have made 

 their way in from the surface of the epiphysis and form a rich 

 network about the cell cords and apparent acini. This onto- 

 genetic differentiation in the two forms just described would 

 certainly seem to indicate a process which had as its object the 

 rich vascularization of discretely outlined epithelial areas. Such 

 a differentiation would seem to adapt itself best to the purposes 

 of internal secretion. 



Marburg 259 shows in the development of the pinoal gland in 

 man histological appearances very closely resembling those 

 illustrated in figures 91, 92, 93, and 94 of the authors (fig. 95). 



