No. 3-] THE VERTEBRATE HEAD. 571 



Fig. 119 is a specimen of the same age as that shown in 

 Fig. 48. It shows that the roof of the thalamencephalon is 

 raised into two nearly equal protuberances. 



Fig. 1 20 shows the beginning of the reduction of the ante- 

 rior of these elevations, and the increased growth of the 

 posterior one. In Fig. 1 2 1 the posterior elevation, or epiphysis, 

 has become a tubular outgrowth, while the anterior one is 

 reduced and depressed. 



In Fig. 122 the tubular stalk of the epiphysis is considerably 

 elongated, while that part of the roof of the thalamencephalon 

 in front of it has become reduced to a narrow fold that has 

 been carried ventrad. The paraphysis has now arisen from 

 the roof of the prosencephalon. The beginning of the choroid 

 plexus extends from the structure down into the ventricle of 

 the fore-brain, and its posterior part is connected with the fold 

 in front of the epiphysis. The anterior and posterior commis- 

 sures are now distinguishable. 



Figs. 123 and 124 exhibit substantially the same relations ; 

 the choroid plexus has considerably increased in extent, and is 

 rapidly becoming much convoluted ; the brain vesicles have 

 become reduced. In Fig. 124 the distal end of the epiphysis 

 is inserted into a cavity in the roof of the cranial wall, that is, 

 the mesenchyme forms a cup over its rounded end, 



3. Comparisons between Epiphysial Outgrowths. 



The recent studies have done much towards establishing a 

 basis for comparison between the epiphysial outgrowths in 

 different groups of animals. Beraneck, Francotte, and others 

 claim that the anterior epiphysial vesicle — commonly desig- 

 nated the pineal eye — is formed independently, having no 

 generic connection with the posterior one, or epiphysis. 

 Hill's observations on the Teleosts coincide with this view. 

 Klinckowstrom, on the other hand, claims that the anterior 

 one is formed at the expense of the posterior one, and substan- 

 tiates his conclusions with figures of sections of Iguana, 

 showing the two vesicles in union with one another. It is not 

 difficult to conceive how the condition represented by Kline- 



