174 DEVELOPMENT DURING THE THIRD DAY. 



embryo. Both the head- and tail-ends of the embryo become 

 quite distinct, and the side-folds make such considerable pro- 

 gress that the embryo is only connected with the yolk by a 

 broad stalk. This stalk is double, and consists of an inner 

 splanchnic stalk, continuous with the walls of the alimen- 

 tary canal, and an outer somatic stalk, continuous with the 

 body-walls of the embryo. The somatic stalk is very much 

 wider than the splanchnic. (Compare fig. 121 E and F, which 

 may be taken as diagrammatic longitudinal and transverse 

 sections of the embryo on the third day.) A change also takes 

 place in the position of the embryo. Up to the third day it is 

 placed symmetrically, on the yolk, with its ventral face down- 

 wards. During this day it turns so as partially to lie on its left 

 side. This rotation affects first the head (fig. in), but in the 

 course of the fourth day gradually extends to the rest of the 

 body (fig. 1 1 8). Coincidently with this change in position the 

 whole embryo undergoes a ventral and somewhat spiral flexure. 



During the latter part of the second day and during the 

 third day important changes take place in the head. One 

 of these is the cranial flexure. This, which must not be con- 

 founded with the curvature of the body just referred to, com- 

 mences by the bending downwards of the front part of the head 

 round a point which may be considered as the extreme end 

 either of the notochord or of the alimentary canal. 



The cranial flexure progresses rapidly, the front-brain being 

 more and more folded down till, at the end of the third day, it is 

 no longer the first vesicle or fore-brain, but the second cerebral 

 vesicle or mid-brain, which occupies the extreme front of the 

 long axis of the embryo. In fact a straight line through the 

 long axis of the embryo would now pass through the mid-brain 

 instead of, as at the beginning of the second day, through the 

 fore-brain, so completely has the front end of the neural canal 

 been folded over the end of the notochord. The commencement 

 of this cranial flexure gives the body of an embryo of the third 

 day somewhat the appearance of a chemist's retort, the head of 

 the embryo corresponding to the bulb. On the fourth day the 

 flexure is still greater than on the third, but on the fifth and 

 succeeding days it becomes less obvious. 



The anterior part of the fore-brain has now become greatly 



