No. 3] THE VERTEBRATE HEAD. 569 



osts, the anterior vesicle of Hill, but this identification is not 

 right. 



We have in this specimen the first appearance of the tubular 

 outgrowth, which, according to previous authors, marks the 

 beginning of the epiphysis. It is somewhat important to fix 

 the stage at which this outgrowth begins. Its very first appear- 

 ance as an elevation of the roof of the thalamencephalon is in 

 the stage represented in Fig. 36 after the saucer-stage of the 

 ear is past. 



In Fig. 49 the posterior protuberance of the roof of the 

 thalamencephalon is assuming a tubular form. It becomes 

 the epiphysis. The surface of the mid-brain is now considera- 

 bly changed. Two furrows have made their appearance, which 

 divide the lateral walls into three nearly equal lobes. These 

 do not show at all from the exterior before the removal of the 

 epidermis and the mesoblast. 



The gradual reduction of the thalamencephalon by compres- 

 sion between the central hemispheres and the mid-brain is 

 shown consecutively in Figs. 53, 54, 55, 57, and 60. In 

 Fig. 54, and later, the epiphysis appears pear-shaped from 

 the side, but from in front the enlargement on the distal end is 

 seen to be broader than thick. It is borne upon a hollow 

 stalk. 



Fig. 57 represents the brain of an embryo considerably older 

 than in Fig. 56. The thalamencephalon is now very much 

 compressed. The anterior half of it, which originally had a 

 rounded protuberance growing from its roof, is now reduced to 

 a small semicircular fold in front of the epiphysis. 



Fig. 59 is a view upon the epiphysis of the same embryo 

 from directly in front. It was obtained by removing the cere- 

 bral lobes, which lie in front of, and partly hide the epiphysis. 

 The epiphysis is seen to be composed of a stalk and an enlarged 

 distal extremity ; leading from the stalk on either side are the 

 epiphysial peduncles. In this figure we are looking through 

 the ventricle of the thalamencephalon into that of the mid- 

 brain. On the inside of the lateral walls of the opening are 

 two enlargements, the beginnings of the thalami optici. These 

 contain the ganglia hebenulse. 



