302 ARTHUR WILLEY. 



have begun to arrange themselves in such a way as to give 

 plainly the appearance of the lumen being surrounded by a 

 distinct epithelium. Fig. 10 shows the free communication 

 between the tube which is being constricted off from the 

 cerebral vesicle, still ending blindly in front, and the cavity of 

 the vesicle itself. The rudiment of the neuro-hypophysial 

 tube or canal has at this stage slightly shifted its relative posi- 

 tion from that which it occupied in the preceding stage, having 

 approached more nearly to the dorsal middle line. This shift- 

 ing can readily be understood by a comparison of fig. 9 with 

 fig. 6, from which it will be seen that during the transition 

 from the preceding stage to the one now under consideration 

 the wall of the cerebral vesicle has become drawn out to a 

 thin membrane in the left latero-ventral region of the vesicle. 

 It will be remembered that the migration of the otocyst was 

 directly traceable to a similar local thinning out of the wall of 

 the vesicle. 



Stage III. — This is the stage at which the communication 

 between the cavity of the nervous system and the base of the 

 stomodseum at the point of junction between the stomodseum 

 and the wall of the branchial sac is effected (see fig. 2 as to 

 depth of stomodseal invagination). 



The tube, whose constriction from the wall of the cerebral 

 vesicle we have been following, has now separated itself en- 

 tirely from the latter (fig. 11), and has meanwhile acquired an 

 opening into the stomodseum (figs. 12, 13). The cerebral 

 vesicle itself has entered upon the process of histolytic disin- 

 tegration which eventually leads to its entire disappearance. 



Thus the neural tube, of which the neuro-hypophysial 

 canal, so called on account of its later destiny, is merely a 

 continuation, now opens into the stomodseum ; but the open- 

 ing is a perfectly simple one at present, and no appreciable 

 evagination from the wall of the stomodseum can be demon- 

 strated. Later on an evagination does possibly take place, 

 and the opening which we see at this stage appears to be 

 carried somewhat further back. The hypophysis has not yet 

 differentiated itself from the nervous system. 



