210 THE HYPOBLASTIC LINING GILL-SLITS. 



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 

 retrogressive metamorphosis. It also becomes bent upon itself 

 in a ventral direction through an angle of 180*^ ; vide PL XIV. 

 fior. 9 a and 16 a. In some cases this curvature is even more 



o 



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 term- 

 inal part acquires by stage a distinct dorsal curvature. 



Hypoblast of the 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 

 tow^ards 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 dissolution of the wall separating the lumen of the throat 

 from the exterior, a free communication from the throat out- 

 wards is effected ; vide PI. xiv. fig. 5 a and h, and 13 6. Thus 

 it happens that the walls lining the clefts are entirely formed 

 of hypoblast. The clefts are formed successively \ the anterior 

 appearing first, and it is not till after the rudiments of three 

 have appeared, that any of them become open to the exterior, 

 i Vide riates vi. and vii. 



