100 THE SECOND DAY. [CHAP. 



embryo (Fig. 28), they are but slightly indicated in 

 hardened specimens (Fig. 27). 



When an embryo of the eaiiy 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 

 Avrinkles 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 constrictions, one separat- 

 ing the first cerebral vesicle from that part of the 

 medullary canal Avhich is immediately behind it, and 

 the other separating this second portion from a third. 

 So that 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 tJiird cerebral vesicle 

 have been added to the first (Fig. 27, mb, hb). They 

 may be also called the " fore brain," the " mid brain," 

 and the "hind brain," for into these parts will they 

 eventually be developed. 



The optic vesicles, lying underneath the epiblast, 

 towards the end of the day are turned back and pressed 

 somewhat backwards and downwards against the sides 

 of the first cerebral vesicle or fore brain, an elongation 

 of their stalks permitting this movement to take place. 

 The whole head becomes in consequence somewhat 

 thicker and rounder. 



Before the end of the day the fore brain elongates 

 anteriorly. The part so established is not at first sepa- 

 rate from that behind, but it is nevertheless the first 



