436 



EMBRYOLOGY. 



the parental tissue are united with its upper end by means of 

 connective tissue. 



In Mammals the development takes place in a manner similar to 

 that of the Chick. In the Rabbit there also arise follicles, each of 



which at first encloses a small 

 cavity, but later becomes solid. 

 They are then entirely filled with 

 spherical cells, which possess a 

 certain resemblance to lymph- 

 corpuscles. The opinion has 

 therefore been expressed by many 

 (HENLE) that the pineal body is 

 a lymphoid organ, an opinion, 

 however, which is refuted by the 

 study of the development, for 

 genetically the follicles are ex- 

 clusively epithelial structures. 



In the adult there are formed 



within the individual follicles concretions, the brain-sand (acervulus 

 cerebri). 



In Man the pineal body, which begins to appear in the sixth week 

 (His), exhibits a peculiarity as regards its position. Whereas the 

 free end of the epiphysis is at first directed forward, and in other 

 Vertebrates is also retained in this position, it acquires in Man an 

 opposite direction, inasmuch as it bends backward on to the surface of 

 the lamina quadrigemina. Probably this is connected with the fact 

 that the gland is crowded back by the excessive development of the- 

 corpus callosum. 



As the signification of the pineal gland is still doubtful, so is that 

 of the pituitary body or hypophysis cerebri, which, as has been 

 previously mentioned, is united with the floor of the between-brain 

 at the apex of the infundibular process. 



Fig. 246. Section through the pineal gland 

 of a Turkey, after MIHALKOVICS. Mag- 

 nified ISO diameters. 



/, Follicle of the pineal gland with its cavities ; 

 6, connective tissue with blood-vessels. 



The Development of the Hypophysis (Pituitary Body}. 



The hypophysis is an organ which has a double origin. This is 

 expressed in its entire structure, since it is composed of a larger, 

 anterior and a smaller, posterior lobe, which in their histological 

 characters are fundamentally different from each other. 



In order to observe the beginning of its formation, it is necessary 

 to go back to a very early stage (fig. 237), in which the oral sinus 



