STAGES B TO G. THE NOTOCHORD. 309 



The Notochord, 



One of the most interesting features in the Elasmobranch 

 development is the formation of the notochord from the hypo- 

 blast. All the steps in the 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 hypoblast retains 

 a perfectly normal structure and uniform thickness throughout. 

 In the next few sections (PI. 10, fig. I c, ch'} a slight thickening is 

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

 lary canal. The layer, which 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 (PI. f 10, 

 fig. I b, c/i). 



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 gradations 

 between complete separation and complete attachment are to be 

 seen. 



Where the separation first appears, a fairly thick bridge of 

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

 but anteriorly this bridge becomes excessively delicate and thin 

 (PI. 10, fig. I a), and in some cases is barely visible except with 

 high powers. 



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



