NOTES ON ELASMOBRANCH DEVELOPMENT. 581 
ConTINUITY OF CELLS AND LAYERs. 
The continuity between the different layers and organs of 
the embryo to which I first called attention in Peripatus is 
found in all Vertebrate embryos that I have examined. In 
fact, there is a network of pale protoplasmic fibres extending 
inwards from the nucleated protoplasm of the various surfaces. 
When this network has nuclei at the nodes, we get the reticu- 
lated tissue, or embryonic mesoderm, or mesenchyme. In 
Scyllium it is at first sparse and without nuclei. In Raja, 
on the other hand, it is very richly developed, and rich in 
nuclei. In Raja, in other words, the protoplasmic connections 
passing between the various organs and layers are very con- 
spicuous and well marked. In Scyllium this tissue is at first 
without nuclei, as I have said. But soon it acquires nuclei and 
becomes denser. Where do the nuclei come from? In my 
opinion they are derived partly from the epithelial walls of 
the somites, partly from the anterior mass of mesoderm in 
which the notochord, gut, &c., ends, and partly from the 
growing tissue of the caudal swellings, and perhaps also from 
the neural crest. 
We now pass on to speak of the neural crest in those 
Elasmobranchs which I have studied. 
The nerve crest was first discovered by Balfour in the trunk 
region of Elasmobranch embryos. Marshall! has observed it in 
the chick, and describes it as occurring in the anterior part of 
the spinal cord region and extending continuously forward 
into the fore-brain. 
Van Wyhe and Kastschenko also both describe the nerve 
crest in the Elasmobranch embryos they examined as reaching 
from the region of the fore-brain continuously backwards. 
The cranial nerves and the posterior roots of the spinal nerves 
grow out from the nerve crest, and the nerve crest persists 
itself in part as the longitudinal commissure. Both Balfour 
and Marshall state that this longitudinal commissure extends 
1 «Quart. Journ, Micr, Sci.,’ vol. xviii. 
