THE ORGANS OF THE OUTER GERM-LAYER. 439 



At an early period an evagination from the between-brain 

 (figs. 247, 249), called the infundibuhim (tr\ has grown out toward 

 the sac of the hypophysis and applied itself to the posterior wall of 

 the latter, which it has folded in toward the anterior or opposite 

 wall. 



This first stage is followed by a second, in which the sac and the 

 adjoining end of the inf undibulum are metamorphosed into the two 

 lobes of the complete organ already mentioned. 



The sac begins (in Man in the second half of the second month, 

 His) to send out from its surface into the surrounding very vascular 

 connective tissue hollow tubules (the tubules of the hypophysis) 

 (figs. 248, 249 hy'). These are then detached from the walls of the 

 sac, by becoming enclosed on all sides by vascular connective tissue. 

 In this respect the process of development agrees in the main with 

 that of the thyroid gland, only that the spherical follicles are here 

 represented by tubular structures. After the entire sac has been 

 resolved into a large number of small, tortuous tubules provided with 

 narrow lumina, the lobe thus produced applies itself closely to the 

 lower end of the inf undibulum, with which it becomes united by 

 means of connective tissue. 



The end of the infundibulum itself is transformed in the lower 

 Vertebrates into a small lobe of the brain, in which, moreover, 

 ganglionic cells and nerve-fibres can be identified. In the higher 

 Vertebrates, on the contrary, no trace of such histological elements 

 can be detected in the posterior lobe of the hypophysis, which in 

 these forms consists of closely packed spindle-cells, and thus acquiros 

 a close resemblance to a spindle-cell sarcoma. 



(5) Development of the First or Fore-Brain Vesicle. 



The most important changes, the comprehension of which is in 

 part attended with serious difficulties, take place in the vesicle of the 

 fore-brain or cerebrum. It is divided (fig. 250), even at the time of 

 its formation, as has already been mentioned, into a right and a left 

 portion, owing to the fact that its wall becomes infolded from in 

 front and from above by means of a vertical process of the connective- 

 tissue envelope of the brain, the primitive falx. The two portions, 

 the vesicles of the hemispheres (hms), come close together, being 

 separated by only the narrow longitudinal or interpallial fissure (msp), 

 which is filled up by the falx, so that their median surfaces become 

 mutually flattened, whereas their lateral and under surfaces are 



