256 ADAM SEDGWICK. 



Continuity of Cells and Layers. 



The continuity between the different layers and organs of 

 the embryo to which I first called attention in Peripatns 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 Raj a, 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 

 ' ' Quart. Journ. Micr. Sci.,' vol. xviii. 



