54 Journal of Comparative Neurology. 



commissure into the stalk of the primary vesicle as shown in 



Fig- 33- 



The secondary vesicle is much as described in the preced- 

 ing stage. As yet we have not observed a well defined lumen. 

 We have noticed, however, a condition similar to that shown in 

 Figs. 20 and 31, where the cells are scattered, and which might 

 be considered the forerunner of the cavity which is about to 

 appear. 



An interesting feature of this stage is the retarded forma- 

 tion of cartilage over the vesicles. At this time the increase 

 in formation of pigment is not only apparent in the skin, but it 

 likewise begins to form in the connective tissue surrounding the 

 vesicles. 



Paraphysis. The paraphysis has increased considerably in 

 size ; its walls still consist of a single layer of cells, the anterior 

 being thicker than the posterior. Antero-dorsally it sends off 

 two diverticula. The larger one extends forward in line with 

 the paraphysis, while the smaller lies to the left and extends 

 laterally. Histologically these diverticula do not differ from 

 the anterior wall of the paraphysis. 



Latva 22-26 days, 15-16 mm. 



With the formation of cartilage the skull begins to take 

 definite shape, and with these changes there arises a consider- 

 able space between the roof of the brain and the skull (Fig. 

 34). The effect of these changes on the lorm of the epiphysis 

 is well shown in Fig, 35, which represents a more highly magni- 

 fied view of the epiphysis in the plane of Fig. 34. The stalk 

 is much longer than in any previous stage and the structure as 

 a whole is inclined slightly forward. The upper portion, or ves- 

 icle proper, forms a continuous oval outline with the stalk. 

 Posteriorly it extends over the stalk, and the whole struc- 

 ture has the general appearance of the upper portion of an 

 interrogation point. Its lumen follows the general outline, is well 

 defined throughout its entire extent, and is still widely open into 

 the third ventricle. 



Transverse sections of this stage are shown in Figs. 37, 38, 



