72 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



forming the lower layer, are of a fairly uniform size, and poly- 

 gonal from mutual pressure. The whole edge of the blastoderm 

 is thickened, but this thickening is especially marked at its 

 embryonic encl. 



This thickened edge of the blastoderm is still more conspi- 

 cuous in the next and second stage (PI. 3, fig. 3). 



In the second stage the chief points of progress, in addi- 

 tion to the increased thickness of the edge of the blastoderm, 

 are 



(1) The increased thickness and distinctness of the epiblast, 

 caused by its cells becoming more columnar, though it remains 

 as a one-cell-thick layer. 



(2) The disappearance of the cells from the floor of the seg- 

 mentation cavity. 



The lower layer cells have undergone no important changes, 

 and the blastoderm has increased very little if at all in size. 



From PI. 3, fig. 3, it is seen that there is a far larger 

 collection of cells at the embryonic than at the opposite end. 



Passing over some rather unimportant stages, I will come to 

 the next important one. 



The general features of this (the third) stage in a surface 

 view are 



(1) The increase in size of the blastoderm. 



(2) The diminution in size of the segmentation cavity, both 

 relatively and absolutely. 



(3) The appearance of a portion of the blastoderm pro- 

 jecting beyond the rest over the yolk. This projecting rim 

 extends for nearly half the circumference of the yolk, but is 

 most marked at the point where the embryo will shortly appear. 

 I will call it the " embryonic rim." 



These points- are still better seen from sections than from 

 surface views, and will be gathered at once from an inspection 

 of PI. 3, fig. 4. 



The epiblast has become still more columnar, and is 

 markedly thicker in the region where the embryo will ap- 

 pear. But its most remarkable feature is that at the outer 

 edge of the "embryonic rim" (e r) it turns round and becomes 

 continuous with the lower layer cells. This feature is most im- 

 portant, and involves some peculiar modifications in the develop- 



