1^6 Journal of Comparative Neurology. 



ued forward movement. In the preceding stage its axis was 

 approximately perpendicular, whereas in the present it is in- 

 clined anteriorly some i 5°. Its cavity is about the same in form 

 and extent. The walls of both stalk and vesicle remain un- 

 changed. The elongated pear-shaped cells lining the cavity 

 appear as shown in Fig. 39. 



The secondary vesicle lies relatively lower and is more ob- 

 scured, owing to the continuous overgrowth of the primary. Its 

 form is that described in the preceding stage, while its volume 

 is somewhat less. Its lumen, which was plainly visible in the 

 preceding stage, is but faintly indicated and in some cases has 

 entirely disappeared, while the elongated cells lining the early 

 cavity are no longer distinguishable. 



The cartilage which in earlier stages was noticeably thinner 

 just above the vesicle has now become of uniform thickness in 

 this region ; the layer of pigment lying just beneath the cartil- 

 age is not yet continuous over the vesicle. 



Paraphysis. The paraphysis has increased greatly in size ; 

 along its middle portion it appears bilobed, while distally it ends 

 in a large median prolongation. There appear at this time 

 numerous foldings and diverticula. 



Summary. 



We confirm the observations of Hill that there are three 

 outgrowths from the roof of the brain, two of which ( pri- 

 mary and secondary epiphyses) arise from the roof of the thal- 

 amencephalon, while the third ( paraphysis ) arises from the 

 roof of the prosencephalon. 



The primary epiphysis first appears in a 3-4 days embryo 

 as a median, posteriorly directed, unpaired evagination. It 

 soon differentiates into a narrow stalk-like portion which proxi- 

 mally passes over into the brain wall, while distally it enlarges 

 to form a vesicle. A well defined lumen extends throughout 

 the structure and opens into the thalamocoele. The walls of 

 the vesicular portion are at first several layers of cells thick. 

 The dorsal wall thins to one or two layers, except in its proxi- 

 mal portion, where it remains thickened. The ventral wall be- 



