DEVELOPMENT OF ELASMOBRAXCH FISHES. 93 



blast. All the steps in ttie process by which this takes place 

 can be followed with great ease and certainty. 



Up to stage B the hypoblast is in contact with the epiblast 

 immediately below the medullary groove, but exhibits no trace 

 of a thickening or any other formation at that point. 



Between stage B and C the notochord first arises. 



In the hindermost sections of this stage the h}^oblast retains 

 a perfectly normal structure and uniform thickness throughout. 

 In next few sections, PL ix. fig. 1 c, c/i', a slight thickening is 

 to be observed in the hypoblast, immediately below the medul- 

 lary canal. The layer, w^hich elsewhere is composed of a single 

 row of cells, here becomes two cells deep, but no sign of a 

 division into two layers exhibited. 



In the next few sections the thickening of the hypoblast 

 becomes much more pronounced ; we have, in fact, a ridge 

 projecting from the hypoblast towards the epiblast (PL IX. 

 fig. Ih, cli). 



This ridge is pressed firmly against the epiblast, and causes 

 in it a slight indentation. The hypoblast in the region of the 

 ridge is formed of two layers of cells, the ridge being entirely 

 due to the uppermost of the two. 



In sections in front of this a cylindrical rod, which can at 

 once be recognised as the notochord and is continuous with the 

 ridge just described, begins to be split off from the hypoblast. 

 It is difficult to say at what point the separation of this rod 

 from the hypoblast is completed, since all intermediate gra- 

 dations betw^een complete separation and complete attachment 

 are to be seen. 



Where the separation first appears, a fairly thick bridge of 

 hypoblast is left connectiug the two lateral halves of the layer, 

 but anteriorly this bridge becomes excessively delicate and thin 

 (PL IX. fig. 1 c), and in some cases is barely visible except 

 with high powers. 



From the series of sections represented, it is clear that the 

 notochord commences to be separated from the hypoblast an- 

 teriorly, and that the separation gradually extends backwards. 



The posterior extremity of the notochord remains for a long 

 time attached to the hypoblast ; and it is not till the end of the 

 period treated of in this chapter that it becomes com>pletely free. 



