398 



is just fairly established. The head-end is already wider than the 

 rest of the embryo, it has begun to show that tendency to broaden 

 that is characteristic of the anterior end of the embryo. The division 

 into segments is most clearly defined in the non-axial part of the 



Fig. 1. Very young embryo of Squalus acanthias (Acanthias vulgaris) showing primi- 

 tive segments in the embryonic rim. 



embryo, but it also extends faintly into the axial part. The non- 

 axial part of the embryo, is that thickened part of the blastodermic 

 rim which extends in unbroken continuity from the embryo, and 

 gradually merges into the germ ring. It represents (if we may ac- 

 cept the view of concrescence) the separated halves of the embryo 

 that have not yet been brought together in the process of embryo 

 formation. It might appear, at first thought, that the folds re- 

 presented in this figure are mechanically produced by the sharp 

 curving of the blastodermic rim to enter the axial embryo. But that 

 such is not the case becomes evident in following the history of 

 these segments. It will be noticed, that the segments are of uniform 

 width and that they extend laterally further than the mechanical in- 

 fluence of the bending of the material that enters the axial embryo: 

 moreover, this same segmented condition may be traced from this 

 stage onward in unbroken continuity until it is clearly defined through- 

 out the length of the embryo. 



In fig. 2, the metameric segmentation extends along the lateral 

 margins of the medullary plate, from the anterior tip of the embryo, 

 backwards to the hinder end. In other embryos of this same age I 

 have been able to trace the segments into the. non-axial part of the 



