ANATOMY AND DEVELOPMENT. 



225 



c.u^ 



by subsequent extension the lumen of the subneural gland 

 and its duct. 



Thus the anterior portion of the primitive neural tube, 

 having become constricted off from the cerebral vesicle 

 of the larva, and having given rise by proliferation from 

 its dorsal wall to the definitive ganglion, becomes bodily 

 converted into that structure which we shall call, in agree- 

 ment with JuLiN, the hypophysis. 



The opening of the latter into the base of the buccal 

 tube becomes the dorsal tubercle of the adult. Finally, at 

 a much later stage, the glandular portion of the hypophy- 

 sis arises by proliferation of 

 spongy tissue from the ven- 

 tral wall of that portion of 

 the neuro-hypophysial tube 

 which lies immediately be- 

 low the ganglion. 



A section through the 

 cerebral vesicle of a larva 

 of Distaplia, a colony-build- 

 ing Ascidian, showing the 

 hypophysis in process of „. ^ t^ , , .• ,k ^, 



J ^ ^ J ^ Fig. 106. — Frontal section through 



being constricted off from cerebral vesicle of a larva of Distaplia 



, . , . . . .^. inagnihirva, to show the origin of the 



the vesicle, is given m hlg. ganglion and hypophysis. (After HJORT; 



106. In this genus the con- <=°™'^i';^'',°" °f '"■°.'^S",<:=-\ , , 



^ In the larva of Distaplia, the hypophy- 



dition of things generally is sis opens into the branchial sac be- 



,._- - , hind tlie stomodo?um. 



very different from what ^„ Cerebral vesicle, ec. Ectoderm. 



obtains in Ciona, but it is ''"• Endoderm. ^4-. Ganglion, hy. Hy- 



pophysis (neuro-hypophysial tube). 



introduced to show the 



essential similarity in the mode of origin of the hypophy- 

 sis in this form, as observed by Dr. Johan Hjort. 



In Distaplia, as is also the case to a less extent in 

 Clavclina, the ganglion begins to develop from the wall 



