NOTES ON ELASMOBRANOH DEVELOPMENT. 255 



organs with which it is at first continuous^ i.e. notochord, 

 ectoderm^ and gut, I should like to point out a resemblance 

 in its early condition to the primitive streak of the Amniota. 

 Like the primitive streak, it is a densely packed mass of nuclei 

 in continuity with all the layers and organs of the hody. 

 The ectoderm, endoderm, notochord, and mesoderm, all are 

 continuous with it ; and as the primitive streak is the 

 growing point for the hind end of the embryo, so it appears to 

 contribute in a similar manner to the front end. 



The Anterior Somites in Raja. — In Raja the segmentation 

 of the anterior mesoderm and the prominence of the first two 

 somites are not nearly so conspicuous as in the other genera. 

 The condition of the anterior mesoderm after its separation 

 from the endoderm is quite different from that in Scy Ilium and 

 Pristiurus. It does not assume the condition of an " epithe- 

 lium " arranged round the cavities or the incipient cavities of 

 somites. On the contrary, it at once assumes the form of 

 " embryonic connective tissue," i. e. of a mass of stellate 

 cells all connected together by their processes. In other 

 words, it at once takes on the form which is only secondarily 

 attained by the same mesoderm of the two other genera after 

 passing through the epithelial condition. This difference in 

 the early structure of the cephalic mesoderm of Raja and 

 Scyllium is another proof, if such were needed, that the 

 distinction between " mesenchyme " and epithelial mesoderm 

 to which the Hertwigs have so prominently called attention 

 has not the importance which they attribute to it. The cavi- 

 ties of the first two somites make their appearance in this 

 stellate mesoderm at about Stages G, H. But they are at first 

 inconspicuous, having the appearance of blood-vessels, and are 

 without the conspicuous epithelial lining. In fact, the cells 

 lining them have at first simply the characters of the reticulate 

 mesoderm tissue, of which, indeed, they are merely a part. 



