580 ADAM SEDGWICK. 
organs with which it is at first continuous, 1.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 body. 
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 Scyllium 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. 
