THE ORGANS OF THE INNEK GERM-LAYER. 



285 



mid-brain. Furthermore the frontal process, in consequence of the curvature, 

 covers in the oral sinus more and more from above and in front, and thereby 

 contributes to its depth. 



As His has shown for the human embryo, the pharyngeal membrane before 

 rupturing extends obliquely backward and upward from the mandibular arch, 

 and becomes firmly attached at the point of curvature ftp, where, as a result of 

 the bending, the anterior and posterior halves of the head meet each other 

 at right angles. Even after the rupture of the pharyngeal membrane there 

 is retained, in front of 



its attachment, a small gh zh *' sf SH 



pit, which constitutes 

 RATHKE'S pocket (fig. 

 153 fy/>- 



It is to be noted that 

 the oral sinus, in front 

 of the pharyngeal mem- 

 brane, and the fore gut, 

 which lies behind it, do 

 not correspond respec- 

 tively to the cavities de- 

 signated in the anatomy 

 of the adult as oral 

 cavity and pharynx. But 

 the region of RATHKE'S 

 pocket, which belongs 

 to the embryonic oral 

 sinus, is in the adult 

 referred to the pharynx. 



In consequence of the 

 early and complete dis- 

 appearance of the pha- 

 ryngeal membrane, it is 

 no longer possible to 

 say at what place in the 

 adult is to be sought 

 the transition from the 



primitive, epidermis-lined oral sinus to the epithelial layer of the alimentary 

 tube. 



B. The Development of the Visceral Clefts. 



While the changes described take place in the vicinity of the oral 

 sinus, several visceral clefts make their appearance immediately 

 behind the jaw-arches upon either side of the body. They are 

 developed in the case of Selachians, Teleosts, Ganoids, and Am- 

 phibia, as well as Amniota, in a rather uniform manner (figs. 154, 

 155). From the epithelium of the fore gut there are formed deep 

 outpocketings (sch 1 scA 6 ), which run from above downward on the 

 lateral wall of the throat parallel to the jaw-arches. They crowd 



ba 



Fig. 153. Median sagittal section through the head of a Chick 

 incubated 4'- days, after MIHALKOVICS. 



SH, Parietal [mid-brain] elevation ; av, lateral ventricle of the 

 brain ; or", third ventricle ; t>*, fourth ventricle ; Sw, aque- 

 ductus SYLVII ; gh, cerebral vesicle ; zh, between-brain 

 [thalamencephalon] ; mJi, mid-brain; kh, cerebellum; 

 zf, pineal process ; hp, hypophysial (or RATHKE'S) pocket ; 

 eft, chorda ; ba, basilar artery. 



