EL A SMOBRANCHII. 



43 



pleted, since all intermediate gradations between complete separation and 

 complete attachment are to be seen. 



Shortly after the separation takes place, a fairly thick bridge is 

 found connecting the two 

 lateral halves of the hypo- 

 blast, but this bridge is an- 

 teriorly excessively delicate 

 and thin, and in some cases 

 is barely visible except with 

 high powers. In some sec- 

 tions I have observed possi- 

 ble indications of the pro- 

 cess like that described by 

 Calberla for Petronyzon, by 

 which the lateral parts of 

 the hypoblast grow in under- 

 neath the axial part, and so 

 isolate it bodily as the noto- 

 chord. 



It is not absolutely 

 clear whether the noto- 

 cliord is to be regarded as 

 an axial differentiation of 

 the hypoblast, or as an 

 axial differentiation of the 

 lower layer cells. 



The facts of develop- 

 ment both in Amphioxus 

 and Elasmobranchii tend towards the former view ; but the nearly 

 simultaneous differentiation of the notochord and the mesoblastic 

 plates lends some support to the supposition that the notochord may 

 be merel)' a median plate of mesoblast developed slightly later than 

 the two lateral plates. 



The alimentary canal or mesenteron was left as a space between 

 the hypoblast and the yolk, ending blindly in front, but opening 

 behind by a widish aperture, the blastopore or anus of Rusconi {vide 

 fig. 19 B). 



The conversion of this irregular cavity into a closed canal com- 

 mences first of all at the anterior extremity. In this conversion two 

 distinct processes are concerned. One of these is a process of folding 

 off of the embryo from the blastoderm. The other is a simple growth 

 of cells independent of any fold. To the first of these processes 

 the depth and narrowness of the alimentary cavity is due ; the second 

 is concerned in forming its ventral wall. The process of the folding 

 off of the embryo from the blastoderm resembles exactly the similar 

 process in the embryo bird. The fold is a perfectly continuous one 

 round the front end of the embryo, but may be conveniently spoken 

 of as composed of a head-fold and two lateral folds. 



Fig. 25. Thkee sections of a Pristiukxjs 

 embryo slightly older than fig, 28 b. 



The sections shew the development of the noto- 

 chord. 



C/t. notochord; Ch'. developing notochord ; mg. 

 medullary groove; Ip. lateral plate of mesoblast; 

 ep. ei)iblast; hy. hypoblast. 



