272 SCOTT. [Vol. I. 



any floor at all, the hind-brain coming up to the constriction 

 immediately behind the infundibulum, and the third nerve aris- 

 ing from the anterior part of the hind-brain. (See Ahlborn's 

 Fig. 2, PI. XIII, Zeitschr. f. wissensch. Zool. Bd. XXXIX.) 

 Now, without discussing the considerations which have led 

 Ahlborn to adopt this view, as they are more or less of an ^ 

 priori nature, it will suffice to say that his interpretation does 

 not at all correspond to the condition found in embryos and 

 young larvae, as in them a vertical constriction runs down the 

 sides of the brain, from the roof, where the dorsal fold, already 

 mentioned, occurs, to the floor, enclosing a ventral section of 

 the mid-brain of very considerable length. As to the third 

 nerve, there can be no doubt that in the Selachians this nerve 

 arises from the mid-brain, and it would require very strong 

 evidence to prove that in Fctrojny:^on it has a -different origin. 

 The ciliary ganglion and its root, whether or not that be the 

 third nerve, undoubtedly arises from the embryonic mid-brain, 

 and sections of embryos demonstrate in the clearest manner 

 that the floor of the hind-brain does not extend to the infundi- 

 bulum. 



If Ahlborn has been led to his view by the fact that the 

 notochord extends as far forward as the infundibulum, it should 

 be remembered that this extension is a secondary matter. Pri- 

 marily, of course, the notochord does not extend beyond the 

 hypoblast, from which it has been derived, but after its separa- 

 tion it grows considerably in advance of the hypoblast. But 

 the anterior end of the hypoblast is in very nearly the same ver- 

 tical plane as the division between the mid and hind-brains 

 (Figs. 2, 3, and 6, PI. VIII). Primarily, at least, the mid-brain 

 is, as Ahlborn calls it, praechordal, though the notochord does 

 eventually extend beneath it for nearly its entire length. 



The mid-brain undergoes comparatively slight changes in the 

 course of larval development. At the time of hatching, its 

 ventricle is large proportionally, and somewhat expanded above. 

 The roof is thin, and two considerable bands of fibres occupy 

 the sides. These fibrous bands increase rapidly at the expense 

 of the cells, which become reduced to a thin layer surrounding 

 the cavity. The ventricle narrows very much except dorsally, 

 where the roof expands and becomes exceedingly thin, and, 

 receiving a large number of blood-vessels, projects in complex 



