276 Frederick Tilney, 



in position; the cephalic extremity is free while the caudal extremity- 

 has come into contact with the now^ enlarged infundibular process [28]. 

 In this wa}^ a new region of the mouth pouch has become juxta- 

 neural in position and this relation will remain permanent in contra- 

 distinction to the dorsal surface of the pouch, whose juxta-neural 

 position, as already stated, is only transient. The new line of con- 

 tact will determine the future pars infundibularis. The cavity enclosed 

 by the rapidly forming ventral surface, the dorsal surface and the 

 extremities, is the primitive ventricle which, as the gland continues 

 to develop, is destined to become the residual lumen. The anläge of 

 the infundibulum and infundibular process has become more definite 

 in its form and relations; it contains a small lumen which communi- 

 cates with the diencephalic cavity. This lumen is the beginning of 

 the recessus infundibuli and recessus processi infundibuli. The portion 

 of the inter-brain floor wdth w^hicli the dorsal surface of the mouth 

 pouch is in contact shows no change, but cephalad of this region the 

 floor rises gradually in a plateau-like elevation which extends forward 

 as far as the optic recess. 



14 mm Cat Embryo no. 210 (plate XIII, fig. 19). The oral eva- 

 gination [25] in this stage has been entirely constricted off from the 

 mouth cavitj'', except for a slender, solid stalk [19] which connects 

 it with the roof of the mouth. The oral pouch has now assumed the 

 general position which it wall ultimately occupy as the pars buccalis. 

 It presents no marked change in the I'elations as observed in the 

 10 mm embryo. Its ventral and lateral surfaces are free while its 

 cephalic extremity has gradually tapered to a neck-like process from 

 which the slender stalk projects to the roof of the mouth. In the 

 region of the caudal extremity, w^hich in the 10 mm stage had at- 

 tained a juxta-neural relation to the infundibular process [28], certain 

 notable changes have occurred. Here, in the apparent response to 

 the marked growth of the processus infundibuli, the caudal surface 

 has been indented and the indenture further accentuated by the de- 

 velopment of two caudal, arm -like projections [25 a] which embrace 

 the infundibular process. From its arrangement in this stage, it is 

 possible to identify in the pars buccalis a body [25h], a neck [25c], 



