MUSEUM OF COMPARATIVE ZOOLOGY. 219 



part with an anterior much-folded epithelial wall, and a posterior not 

 folded and thinner epithelial wall ; an epiphysial vesicle ; a blood sinus ; 

 and a string of connective tissue extending from the distal end of the 

 vesicle and blood sinus to the region of the parietal vesicle. In the 

 anterior wall of the proximal portion the folding extends down some- 

 what nearer to the brain than is the case in P. Douglassii, and just at 

 its junction with the brain a large blood-vessel is found filled with 

 blood corpuscles (Fig. 12, cp. sng.). As already said in describing the 

 posterior wall in P. Douglassii, the connection (opposite the letters 

 vnt. z ) with the brain is here complete and very evident, though the roof 

 of the third ventricle (tci. thl. opt.) appears in the section to constitute 

 a part of this wall. 



The cells composing the walls of the proximal part are about two or 

 three deep, but not arranged in layers. They are small, distinctly nu- 

 cleated, and the nuclei are apparently perfectly round. They stain 

 readily. On the outer surface of this wall is found, throughout most 

 of its extent, a very thin layer of tissue, the cells of which are much 

 flattened. This layer becomes continued from the apex of the epiphysis 

 as the connective-tissue string (con't. tis.) already mentioned as passing 

 to the region of the eye ; another portion of it also becomes continuous 

 with the pia mater of the brain. 



Figure 10 represents a section through the longest portion of the 

 epiphysial vesicle. In this plane the proximal portion of the epiphysis 

 has not yet appeared, and is not found till we pass to a section in 

 which the long axis of the vesicle has become considerably shortened. 

 In the wall of the vesicle three zones or layers are found. The external 

 one is similar to — in fact, on the posterior surface is continuous with 

 — the thin external layer mentioned in the proximal portion. The 

 second zone, comprising more than half of the entire thickness of the 

 wall, is composed of cells apparently of the same nature as those 

 described as forming the chief portion of the wall of the proximal part ; 

 but the layer is considerably thicker here than there, and on the whole 

 rather more compact (e'th. t Figs. 10 and 11). The third and most 

 internal zone is a deeply pigmented one (pig.). This pigment is so 

 dense that when destroyed no distinguishable structure remains. In 

 the presence of this pigment the species now under consideration differs 

 entirely from P. Douglassii, where no pigment in this region is found. 

 Again, however, attention is called to the fact that great importance 

 cannot be attached to the presence or absence of pigment, Figure 1 1 

 shows the relation between this vesicle and the proximal portion of the 



