310 ARTHUR WILLET. 



rior to the region from which fig. 31 was taken, and shows 

 again the posterior communication between the two portions 

 of the neural tube. In comparing figs. 31 and 32 with figs. 

 28 and 29 of the preceding stage, it will be seen that the hinder 

 opening of the hypophysial portion of the neural tube into that 

 division of it which corresponds to the later cerebral vesicle 

 lies somewhat farther backwards in the present stage ; whereas 

 in fig. 29 the two halves of the neural tube are in open com- 

 munication in the region of the otocyst, in fig. 31 they are dis- 

 tinct from each other in this region, and their lumina unite 

 more posteriorly (fig. 32). This fact illustrates the gradual 

 constriction of the hypophysial tube from the neural tube, 

 which is taking place from before backwards. 



As we trace the series of sections backwards it is found that 

 the lumen of the nerve-tube gradually becomes again narrower, 

 showing that its expansion in the transverse direction is con- 

 fined to a particular region, namely, the region from which 

 the hypophysis takes its origin. 



Stage IV. — A-t this stage the larva possesses a mouth. 

 Fig. 33 is a section taken through the cerebral region of the 

 neural tube of a young larva of this stage at the time of the 

 first formation of the mouth. It would seem that almost im- 

 mediately after the mouth has broken through, a communica- 

 tion is established between the neuro-hypophysial canal and 

 the cavity of the branchial sac at the base of the stomodeeum. 

 This communication is at first a perfectly plain one, and not 

 involved with any evagination from the wall of the branchial 

 sac. 



As to the region of the branchial sac into which the neuro- 

 hypophysial canal opens, it is only reasonable to suppose that 

 it corresponds to the base of the stomodaeal involution. This 

 also follows from a comparison between the depth of the stomo- 

 dseal invagination before the actual perforation of the mouth 

 and the level at which the communication between the neuro- 

 hypophysial canal and the branchial sac becomes effected at a 

 later stage (cf. PI. 10, figs. 2 and 13). 



It is possible that in Clavelina, asinCiona intestinalis, 



