THE ORGANS OF THE OUTER GERM-LAYER. 



437 



has just arisen and is still separated from the cavity of the head-gilt 

 by means of the pharyngeal membrane (rh). At this time the 

 cephalic flexure of the brain-vesicles has already appeared, and the 

 anterior end of the chorda dorsalis (ch) terminates immediately 

 behind the attachment of the pharyngeal membrane. In front of 

 this is located the important place where the hypophysis is developed, 

 as was first established by GOETTE and MIHALKOVICS. The hypo- 

 physis is therefore a product of the outer germ-layer and not a growth 

 from the cavity of the head- gut, as had always been maintained 

 previous to this time. 



The first steps introductory to the formation of the hypophysis 

 take place soon after the rupture of the pharyngeal membrane 

 (figs. 238, 247), some unimportant remnants of which are retained 

 at the base of the skull as the so-called primitive velum palatinum. 

 Anterior to these there is now developed (in the Chick on the 

 fourth day of incubation, in Man during the fourth week, His) a 

 small evagination, the pouch of RATHKE or the pocltet of the hypophysis 

 {/*?/), which grows to- 

 ward the base of the f ' 

 b e t w e e n-b rain (tr\ / / 

 Then it becomes deeper, /*/ ^ 

 begins to be constricted 

 off from its parent tissue, 

 and to be metamor- 

 phosed into a small sac, 

 whose wall is composed 

 of several layers of cylin- 

 drical cells (fig. 248). 



The sac of the hypo- 

 physis (//) remains for 

 a long time in connec- 

 tion with the oral cavity 

 by means of a narrow 

 duct (hyg). In later 

 stages, however, the 

 connection in the 

 higher Vertebrates is 



interrupted, because the embryonic connective tissue, which supplies 

 the foundation for the development of the skeleton of the head, 

 becomes thickened and crowds the sac farther away from the oral 

 cavity (figs. 248, 249). When, later on, the process of chondrification 



\ 



Fig. 247. Median sagittal section through the hypophysis 



of a Rabbit embryo 12 mm. long, after MIHALKOVICS. 



Magnified 50 diameters. 

 tr, Floor of the between-brain with the infundibulum ; 



nh, floor of the after-brain ; ch, chorda ; hy, pocket 



of the hypophysis. 



