IV.] THE OPTIC VESICLES. 77 



of the eyes ; the median one still retains the name of the first 

 cerebral vesicle. The constriction takes place chiefly from 

 above downwards, so that the optic vesicles soon appear to 

 spring from the under portions of the cerebral vesicle. 



The original vesicle being primarily an involution of the 

 epiblast, the walls of all three vesicles are formed of epiblast ; 

 all three vesicles are likewise covered over with the common 

 epiblastic investment which will eventually become the 

 epidermis of the skin of the head. Between this superficial 

 epiblast and the involuted epiblast of the vesicles, there 

 exists a certain quantity of mesoblast to serve as the material 

 out of which will be formed the dermis of the scalp, the 

 skull, and other parts of the head. At this epoch, however, 

 the mesoblast is found chiefly underneath the several vesicles. 

 A small quantity may in section be seen at the sides ; but 

 at the top the epidermic epiblast is either in close contact with 

 the involuted epiblast of the cerebral and optic vesicles or 

 separated from it by fluid alone, there being as yet in this 

 region no cellular elements between the two representing 

 the mesoblast. 



The constrictions marking off the optic vesicles take 

 place of course beneath the cjmmon epiblastic investment, 

 which is not involved in them. As a consequence, though 

 easily seen in the transparent fresh embryo (Fig. 22), they 

 are but slightly indicated in hardened specimens (Fig. 21). 

 In sections they are very clearly seen. 



13. When an embryo of the early part of the second day 

 is examined as a transparent object, that portion of the 

 medullary canal which lies immediately behind the first 

 cerebral vesicle is seen to be conical in shape, with its walls 

 thrown into a number of wrinkles. These wrinkles may 

 vary a good deal in appearance, and shift from time to time, 

 but eventually, before the close of the second day, after the 

 formation of the optical vesicles, settle down into two con- 

 strictions, one separating the first cerebral vesicle from that 

 part of the medullary canal which is immediately behind 

 it, and the other separating that second portion from a third. 

 So, instead of there being one cerebral vesicle only, as at the 

 commencement of the second day, there is now, in addition 

 to the optic vesicles, a series of three, one behind the other ; 

 a second and third cerebral vesicle have been added to the 



