218 BULLETIN OF THE 



a low, broad cone, a condition which gives quite a different general ap- 

 pearance to the sections in the two species. In P. coronata the vesicle 

 is situated somewhat nearer the external surface of the cranial wall than 

 in P. Douglassii ; and the intervening connective tissue differs less, 

 both as regards the fineness and direction of its fibres, from the adjacent 

 tissues, than in the case of P. Douglassii. The vesicle, with its con- 

 nective-tissue capsule, protrudes into the bottom of the pit considerably. 

 The pit is bridged over by the dura mater of the brain, and thus a 

 chamber is formed in which a great quantity of blood corpuscles is 

 found {cp. snff.). It will be remembered that no such blood sinus in 

 this region exists in P. Douglassii, but that numerous blood-vessels do 

 occur here. In P. coronata, however, the sinus replaces the ves^ls. 



2. The Parietal Vesicle. — With regard to the vesicle itself, the only 

 points in which it differs very essentially from that found in P. Douglassii 

 are the absence of the cavities in the retina regarded as blood-vessels, and 

 the far less perfect development of the structures projecting from the 

 internal surface of the retina into the cavity of the vesicle. The latter 

 difference I am inclined to think duo to the probably somewhat greater 

 degree of degeneration of the retinal cells which secrete these struc- 

 tures. That this portion of the retina is more degenerated in P. coro- 

 nata may be supposed from the fact that we find liere considerably 

 more pigment than in the corresponding region in P. Douglassii. How- 

 ever, too much stress must not be laid on the greater or less quantity 

 of pigment, since the quantity is quite variable even within the same 

 species. In one individual of this species pigment was found, though 

 in small quantity, in the lens. 



3. The Epiphysis. — Although this structure does not differ in any 

 essential particular from what we have already seen in the preceding 

 s|)ecies, the fact that several of the points which go to make the study 

 of the epiphysis of much interest are here well brought out, has made it 

 seem best to describe and illustrate the organ in detail. Figures 10, 11, 

 and 12 (Plate IV.) present vertical longitudinal sections from the same 

 animal at different planes to the left of the median plane, Figure 12 

 being very nearl}' median, and Figure 10 farthest removed from it. It 

 should here be said, however, that the sections are not quite vertical; 

 so that, while the epiphysial vesicle is situated more to the left than to 

 the right side of the sagittal plane, yet it is less so than would be 

 inferred from the way in which it appears in the figures. The form 

 of the epiphysis, as a whole, is nearly the same as that found in P. 

 Douglassii, and it is composed of the same parts ; — namely, a proximal 



