7G THE SECOND BAY. [CHAP. 



(Fig. 21, t) makes its appearance, elevating the tail above the 

 level of the blastoderm in the same way that the head was 

 elevated. Lateral folds also, one on either side, soon begin 

 to be very obvious. By the progress of these, together with the 

 rapid backward extension of the head-fold and the slower 

 forward extension of the tail-fold, the body of the embryo 

 becomes more and more distinctly raised up and marked 

 off from the rest of the blastoderm. 



11. The medullary canal closes up rapidly. The wide 

 sinus rhomboidalis becomes a narrow fusiform space (Fig. 21, 

 s.r.), 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. 



12. In the region of the head most important 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. This, from the very first broader 

 than long, now increases so much in breadth as to give the 

 embryo a hammer-headed appearance. The lateral portions, 

 continuing to enlarge, become after a while separated by 

 constrictions from the central portion. The single vesicle 

 is thus converted into three vesicles : a median one connected 

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

 lateral vesicles are known as the optic vesicles (Fig. 21, op. v t 

 Fig. 22, a), and will afterwards become converted into parts 



Head of a Chick at the End of the Second Day vtewed from below 

 as a Transparent Object. (Copied from Huxley). 



/. first cerebral vesicle, a. optic vesicle, d. infundibulum. 



The specimen shews the formation of the optic vesicles (a), as outgrowths 

 from the ist cerebral vesicle or vesicle of the 3rd ventricle, so that the optic 

 vesicles and vesicle of the 3rd ventricle at first freely communicated with each 

 other, and also the growth of the lower wad of the vesicle of the 3rd ventricle 

 into a process which becomes the infundibulum (d). 



