saegent: the onic eeflex appaeatus of veetebeates. 171 



velopment, but the habenular axons, developing at the normal time, are 

 for a period tlie only constituents of Reissuer's fibre, and not until just 

 before birth are they joined by the axons from the optic centres. 



As the embryos have a considerable power of movement within the 

 mother at even an early stage, we may readily imagine that this incom- 

 plete fundament of Eeissner's fibre functions at an early stage as a direct 

 path for reflexes between the habenular (olfactory 1) centres and the 

 musculature. 



b. Adult. (1) Morpholoijy of the Mid-hrain Roof. In Squalus acan- 

 thias the optic lobes when viewed from above are semicircular in 

 outline ; in Alustelus canis they are larger and elliptical, the increase in 

 size having resulted in a bulging forward of the lobes. A sagittal sec- 

 tion through the brain of Mustelus (Plate 2, Fig. 9) shows the arched 

 roof of the mesencephalon carried forward and projecting above the dien- 

 cephalon. In the median plane the posterior commissure is not sepa- 

 rated from the tectum as in Raja, but passes through its anterior portion 

 (Figs. 9 and 10). The more intimate consolidation of the posterior 

 commissure and the tectum opticum has resulted in a great modification 

 of the recess, — which in Raja extends above the commissure, — amount- 

 ing almost to its obliteration (compare Fig. 10 with Plate 3, Fig. 16, 

 rec. ms'coeJ.). A careful study of the figures suggests strongly that the 

 condition in Raja is a degenerate one, — perhaps a return to the primi- 

 tive condition of Petromyzon, — due to the shrinking of the tectum, as 

 the visual apparatus has decreased with the changed habits of this group 

 of fishes. In Mustelus the ependymal thickening (e^end.) has attained 

 a most conspicuous development (Plate 2, Figs. 9, 10, 14). Perhaps 

 its increase in thickness and lateral extent is due to its longitudinal 

 restriction by the obliteration of the recess above the posterior com- 

 missure. Under the commissure it has in cross-section a horseshoe 

 form standing out prominently from the ventricular wall (Fig. 14). 

 It forms an almost complete tube, a longitudinal slit in its ventral wall 

 extending from the anterior limit of the posterior commissure to the 

 ganglia habenulae and the base of the epiphysial stalk. A parasagittal 

 section consequently cuts the ependymal wall of the tube-in two places. 

 In Figure 10 a long stretch of the ventral lip of this tube is shown with 

 an adlierent portion of the ventricular wall (ff) tangentially cut. Fig- 

 ure 9 is a median sagittal section, except under the middle portion of 

 posterior commissure, where the plane of section being somewhat lateral 

 cuts the ventral lip of the ependyma (|3). This median section shows 

 behind the commissure the greatly reiluced recessus (Fig. 9, rec. ms'coel.) 



