No. 3] EPIPHYSIS IN COREGONUS ALBUS. 507 



surface convex ; its ventral slightly concave, and closely applied 

 to the roof of the brain. It is separated .from the epiblast (Ep) 

 by intervening mesenchyme. Along its middle there is a narrow 

 cavity that communicates with the cavity of the brain, slightly 

 to the left of the median plane. The portion (E) of the posterior 

 vesicle is entirely free from the brain wall (B), and median in 

 position. It is a solid mass of cells from the anterior wall of 

 the posterior vesicle. 



The stage just described shows the epiphysial vesicles ten 

 days after their first appearance. The posterior vesicle appeared 

 about two days before the anterior one, and could be seen in 

 the living embryo as a small circle in the roof of the brain, in 

 the median line. When the anterior vesicle appeared, its 

 diameter was three-fourths the diameter of the posterior one, 

 while it had the same position as shown in Fig. I. I was unable 

 to study the anterior outgrowth in the living embryo for a 

 longer period than twenty-five days, as at the end of that time 

 it was so covered by the growth of the posterior vesicle and the 

 surrounding brain wall that in a surface view of the brain it was 

 entirely hidden. 



Sections of stages earlier than that represented in Fig. I. show 

 that these epiphysial vesicles arise as separate outgrowths from 

 the roof of the brain. In each vesicle there is a cavity which 

 opens separately into the cavity of the brain. In stages ten days 

 earlier than that shown in Fig. I. there is, in longitudinal sec- 

 tions, an average of four sections that intervene between these 

 openings, while in the transverse sections there is an average of 

 five sections. Thus the opening of the anterior vesicle is a 

 little more in front of the opening of the posterior vesicle than 

 to the left of it. In stages ten days older than shown in Fig. I. 

 these openings were obliterated, so that I was unable to establish 

 their relation to one another. 



From the study of sections it appears that the anterior vesicle 

 shows an increase in size for about twenty days, and after that 

 a decrease, while the posterior vesicle shows from the begin- 

 ning a gradual increase. The following table is made from 

 transverse sections, and gives the dimensions of the two vesi- 

 cles in three different planes, each taken at right angles to the 

 others : — 



