5o8 



HILL. 



[Vol. V. 



The first column under the anterior vesicle gives the length 

 of the dorso-ventral diameter of the anterior vesicle. The 

 second column gives the distance between the extreme left 

 border of the anterior vesicle and the median plane of the brain. 

 The measurements in the third column of the anterior vesicle 

 are calculated from the number of transverse sections in which 

 the vesicle appears. 



The first column under the posterior vesicle gives the length 

 of a vertical line drawn from the dorsal border of the posterior 

 vesicle to the ventral border of the dorsal brain wall. The 

 second column gives the length of the transverse diameter of 

 the posterior vesicle. The third column is obtained in the same 

 way as that for the anterior vesicle. 



Besides this decrease in size of the anterior vesicle, its cells 

 in older stages are more closely packed than in earlier stages, 

 and show no karyokinetic figures. In early stages karyokinesis 

 is frequently seen. I did not succeed in rearing embryos beyond 

 144 days, but, from the facts stated above, it seems very probable 

 that this vesicle ultimately disappears. 



In embryos seventy-five days old, the posterior epiphysial 

 vesicle begins to grow anteriorly, so that the anterior vesicle is 

 soon pushed to the left of the posterior one, and lies packed 

 between it and the left brain wall. At the same time the fibres 

 of the posterior commissure make their appearance just posterior 

 to the union of the posterior vesicle with the brain roof. A 

 little later there is a folding down of the brain roof anterior to 

 these vesicles. This folding forms the posterior wall of the 

 cerebrum, so that the epiphysial vesicles are clearly seen to be 

 connected with the roof of the thalamencephalon. 



Leydig ('90), in his recent paper, describes in Lacertilia two 

 epiphysial outgrowths. He says the two lie in the median 

 plane, the one just in front of the other ; and that in the embry- 



