STUDIES ON THE PEOTOCHOEDATA. 301 



crescent-shaped cell. Kowalevsky says that the nucleus dis- 

 appears entirely, and the whole cell becomes strongly refractive. 

 The latter is of course true, but the nucleus apparently does 

 not vanish, although it is impossible to see it in the fresh state. 



I should add that I have examined sections of these early 

 stages taken in three planes, but have never found an opening 

 from the neural tube into the stomodseum in the free-swimming 

 larva. 



Stage II. — Fig. 9 represents a transverse section through 

 the region of the cerebral vesicle in a larva of the stage imme- 

 diately preceding fixation. The shape into which the wall of 

 the branchial sac is thrown by the pressure of the superin- 

 cumbent expanded cerebral vesicle should be noted. The epi- 

 thelium of the dorsal wall of the branchial sac is very flat as 

 compared with that of the lateral walls into which it gradually 

 passes on either side. The ventral groove of the branchial 

 sac in fig. 9 is not the endostyle, as follows clearly from a 

 comparison of the actual position of the endostyle as seen in 

 surface views (cf. figures accompanying I) ; but it is possible 

 that Kowalevsky mistook it for a continuation of the endo- 

 style — a mistake which is not diflScult to make if larvae of a 

 later stage are not examined for comparison^ since, after the 

 withdrawal of the tail, both the enteric and the body cavities 

 undergo a general distension, which renders the internal struc- 

 ture and the topographical relation of parts much clearer. 



With regard to the cerebral vesicle, the chief point of dif- 

 ference between this and the preceding stage lies in the fact 

 that the tube which we saw embedded in the thick wall of the 

 vesicle, in the form of a minute cul-de-sac, communicating at 

 its hinder end with the cavity of the vesicle, has now begun to 

 set itself oflF more distinctly from the rest of the wall of the 

 vesicle, forming a considerable prominence slightly to the left 

 of the middle line. The process of constriction by which the 

 tube, or, as it may at once be called, the neuro-hypophysial 

 canal, comes to be entirely separated from the cerebral vesicle 

 has therefore now commenced. It can be noticed in fio-. 9 

 that the nuclei in the neighbourhood of the tube in question 



