100 



DEVELOPMENT OF THE MOUTH. 



The stomodaeum deepens at its upper and anterior part, where it forms a 

 pocket-like protrusion, which grows a certain distance into the angle formed by the 

 sharp bend which the hinder part of the fore-brain now makes with the mid-brain. 



Fig. 117. PROFILE VIEW OP A HUMAN EMBRYO OP ABOUT 15 DAYS, WITH THE ALIMENTARY CANAL 



SHOWN IN LONGITUDINAL SECTION. (His.) 



Fig. 118. SIMILAR VIEW OF A SOMEWHAT OLDER EMBRYO. (His.) 



1, 2, 3, 4, 5, are opposite the respective secondary cerebral vesicles ; from the side of the fore-brain 

 the primary optic vesicle is seen projecting ; ot. otic vesicle ; p.v., septum between mouth and pharynx 

 (primitive velum) ; I, commencing liver in septum transversum ; v, vitelline stalk ; all, allantois enclosed 

 within allantoic stalk ; j v., jugular vein ; c.v., cardinal vein ; s.r., sinus venosus within septum trans- 

 versum ; u.a., umbilical (allantoic) artery ; l.u.v., left umbilical vein. The sharp curve of the trunk of 

 the embryo towards the yolk-sac is normal at this stage. 



In fig. 117 the otic vesicle is still open, and there are only two aortic arches ; in fig. 118 the otic 

 vesicle is closed ; there are now five aortic arches. The primitive velum has disappeared. 



This pocket (Rathke) is the hypophysis cerebri, or pituitary involution of the buccal 

 epiblasf,, and comes presently into connection with the infundibular protrusion of the 

 neural epiblast, the two together forming the pituitary body (see p. 68). It lies 

 just above and in front of the pharyngeal septum. 



The remains of this septum (when it has become broken through to allow of a communica- 

 tion between stomodasum and fore-gut), have been termed theprimitirc rt'litm,\mi the septum 

 has nothing 1 whatever to do with the formation of the permanent velum palati, or with the 

 isthmus of the fauces. The plane of the septum forms in fact an angle with the plane of the 

 future isthmus faucium, so that the primitive mouth or stomodaeum does not by any means 

 correspond with the permanent mouth. In fact the floor of the mouth, including the tongue, 

 is developed behind the septum, and therefore in connection with the fore-gut rather than with 

 the stomodseum, whereas the uppermost part of the pharynx, including the choanse, is hi front 

 of the septum, and therefore belongs to the stomodaeum. 



