MM: ORG kN8 OP Tin: 



OEEM-LAYKB, 



2*5 



mid-brain. Furthermore tin- front;. .uconseqii'' . ature, 



covers in the oral sinus more and more from above and in front, and tl . 

 contributes to its ilcptb. 



As His ha- shown for the human embryo, tin pharyniri-al membrane \> 

 rupturing extends obliquely backward and upward from the raandibular 

 ami bcconu-s firmly attached at the point of curvatu nit of 



tin- bcnuing, the anterior and posterior halves of the head meet each other 

 at rijjht angles. Kven after the rupture of the j.lnryn-.-al membrane there 

 is retained, in front of 



its attachment, a small gh zfi v' :/ 6U 



pit, which constitutes 

 RATHKK'S pocket (tip. 

 i.v; ///;). 



It is to be noted that 

 the oral sinus, in front 

 of the pharyngeal mem- 

 orane. 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 lonc-er 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 Aniniota. in a rather uniform manner (ligs. 154, 

 155). From the epithelium of the fore ^ut thrre are formed deep 

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

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



fta 



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

 incubated 4\ days, aftrr MIHALKOVICS. 



.<//. IVuu-tul [mid-brain] elevation ; ,*c. l.itciul M'litiiclo of the 

 brain ; v 3 , third ventricle ; r*, fmir.h vrr.tricle ; Sic, aque- 

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

 [thalamencephalon] ; iA. inid-hrain ; kh, cerebellum; 

 zf, pineal pivcess ; hj>, liyiM.nhv.-ial (r U^THKK'S) pocket ; 

 ch, chorda; ba, b;vsilar .< 



