THE GILL-SLITS. 433 



Notochord in the Head. 



The notochord during stage G is situated for its whole length 

 close under the brain, and terminates opposite the base of the 

 mid-brain. As the cranial flexure becomes greater and meso- 

 blast is collected in the angle formed by this, the termination of 

 the notochord recedes from the base of the brain, but remains 

 in close contact with the front end of the alimentary canal. At 

 the same time its terminal part becomes very much thinner than 

 the remainder, ends in a point, and exhibits signs of a retro- 

 gressive metamorphosis. It also becomes bent upon itself in a 

 ventral direction through an angle of 180; vide PI. 15, figs. 90 

 and 16 a. In some cases this curvature is even more marked 

 than is represented in these figures. 



The bending of the end of the notochord is not directly 

 caused by the cranial flexure, as is proved by the fact that the 

 end of the notochord becomes bent through a far greater angle 

 than does the brain. During the stages subsequent to K the 

 ventral flexure of the notochord disappears, and its terminal 

 part acquires by stage O a distinct dorsal curvature. 



Hypoblast of tlie Head. 



The only feature of the alimentary tract in the head which 

 presents any special interest is the formation of the gill-slits and 

 of the thyroid body. In the present section the development of 

 the former alone is dealt with : the latter body will be treated 

 in the section devoted to the general development of the ali- 

 mentary tract. 



.The gill-slits arise as outgrowths of the lining of the throat 

 towards the external skin. In the gill-slits of Torpedo I have 

 observed a very slight ingrowth of the external skin towards 

 the hypoblastic outgrowth in one single case. In all other cases 

 observed by me, the outgrowth from the throat meets the 

 passive external skin, coalesces with it, and then, by the dis- 

 solution of the wall separating the lumen of the throat from the 

 exterior, a free communication from the throat outwards is 

 effected ; vide PI. 15, figs. 5 a and b, and 13 b. Thus it happens 



