STUDIES ON THE PKOTOCHORDATA. 309 



This double character of the neural tube only extends in 

 this stage through two or three sections. Anterior and pos- 

 terior to this region it is a simple tube with a single lumen 

 (figs. 27 and 29). Of the two halves of the neural tube in fig. 

 28, that on the left side (right of the figure) retains approxi- 

 mately its present shape, and forms part of the future 

 hypophysis; while the other (right) division of the neural tube, 

 which is at present rather smaller than its neighbour, becomes 

 enormously distended in the later stages, and is converted 

 into the spacious cerebral vesicle. 



We see, therefore, at this stage the first commencement of 

 the separation of the hypophysis from the rest of the larval 

 nervous system taking place entirely independently of any 

 evagination from the wall of the stomodseum, which, indeed, 

 does not yet exist. It is to be noted also that the formation 

 of the hypophysis commences here at a much earlier stage than 

 in the case of Ciona, a fact which is in thorough keeping with 

 the general character of the development of Clavelina, to which 

 I have already alluded. 



The neuro-hypophysial tube decreases considerably in dia- 

 meter in the later stages, owing to the absorption of the yolk 

 with which its cells are at first filled (cf. figs. 28, 31, and 40). 



Stage III. — At this stage the lumen of that portion of the 

 neural tube which will give rise to the cerebral vesicle is com- 

 mencing to enlarge (fig. 31). The neuropore is closed, but 

 there is still no mouth. The lumen of the neural tube in front 

 has a more or less round outline, but widens out transversely 

 behind until, as in the preceding stage, but now in a more 

 pronounced way, it becomes divided into two. This condi- 

 tion is shown in fig. 31, from which it will be seen that the 

 two portions of the neural tube have now reversed the relative 

 dimensions which they held in the preceding stage, — the one on 

 the right side, namely, the one that will become, and, in fact, 

 is becoming, the cerebral vesicle, and which contains the oto- 

 cyst, being considerably larger than the other division of the 

 tube, which, as we have already seen, is the rudiment of the 

 hypophysis. The section drawn in fig. 32 lies slightly poste- 



