306 ARTHUR WILLEY. 



(fig. 23). This temporary obliteration of the lumen of the 

 hypophysis at this point and at this period of the development 

 seems to be a constant feature, and extends over one, or at 

 most two sections of a thickness of about 7 fx. The lumen 

 then opens out again (fig. 23), and in the posterior region of 

 the hypophysial tube^ which now lies closely applied to^ but 

 at the same time distinct from, the ganglion, glandular tissue 

 is seen to be developing from its ventral walP (fig. 24). Here 

 and there the peripheral nuclei of the ganglion are absent in 

 the region where the hypophysis is in contact with the latter 

 (fig. 24). 



We see, therefore, that the hypophysis and the ganglion, 

 which have been gradually difi'erentiating themselves from the 

 common neuro-hypophysial tube, have at last separated entirely 

 from one another by a completion of the constriction of which 

 we saw the commencement in the preceding stage (fig. 17). 



From the appearances presented I am disposed to believe 

 that the anterior portion of the hypophysis, including the 

 funnel-shaped dilatation and the duct, as far as the above- 

 mentioned point of reduction of the lumen, is derived from a 

 secondary evagination from the stomodseal region of the wall 

 of the branchial sac ; while the division of the hypophysis which 

 lies behind that point, and from which the gland is developed, 

 represents the tube derived by constriction from the cerebral 

 vesicle of the larva in the way described above. The original 

 opening, therefore, of the neuro-hypophysial tube into the 

 branchial sac has on this supposition been carried backwards 

 by a secondary outgrowth from the stomodaeum. It is difficult 

 to bring other than circumstantial evidence in support of this 

 view, but it may be possible to test its truth on a future occa- 

 sion. Meanwhile this seems to be a reasonable explanation of 

 the local and temporary obliteration of the lumen which divides 

 the proximal from the distal or glandular portion of the 

 hypophysis. 



^ Seeliger (' Jenaische Zeitschrift,' xviii, p. 100) described the liypopbysis- 

 gland in Clavelina as arising by the aggregation of free mesoderra-cells. He 

 does not, however, commit himself unreservedly to this view. 



