CHAP. V.] THE BRAIN. 97 



and at the end of this period is entirely roofed over. 

 The conversion of the original medullary groove into 

 a closed tube is thus completed. 



The brain. In the region of the head most im- 

 portant changes now take place. We saw that at the 

 beginning of this day the front end of the medullary 

 canal was dilated into a bulb, the first cerebral vesicle, 

 which by budding off two lateral vesicles became con- 

 verted into three vesicles : a median one connected 

 by short hollow stalks with a lateral one on either side. 

 The lateral vesicles known as the optic vesicles (Fig. 

 27, op. v, Fig. 35, a), become converted into parts of the 

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

 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 in addition 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 invo- 

 luted 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 (Fig. 30). 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 between 

 the two no cellular elements representing the mesoblast. 



The constrictions marking off the optic vesicles also 

 F. & B. 7 



