Notes on Elasmobrancli Development. 



By 



Adam Sedgwick, M.A., F.R.S., 



Fellow and Lecturer of Trinity College, Cambridge. 



With Plate XVI. 



Contents. 



PAGE 



1. On the Formation and Growth of the Embryo and on the Blastopore 234 



2. On the Formation of the Mouth and Gill-clefts . . . 245 



3. Segmentation of the Cephalic Mesoderm and Development of Nerves 247 



1. On the Eormation and Growth of the Elas- 

 mobranch Embryo. 



My observations on this subject, which were made upon the 

 genera Scyllium and Raia, have led me to conclusions which 

 diflfer in some respects from those of previous observers. In 

 some of the points with regard to the tail I have been 

 anticipated by Schwarz (' Zeit. f. wiss. Zool./ Bd. xlviii, p, 

 191), Kowalevsky, and Kastschenko ('Anat. Anzeiger,' 3); 

 but as Schwarz's account — excellent though it is — does not go 

 over the whole ground, and Kastschenko's is without figures, 

 while Kowalevsky's is inaccessible, being published in Rus- 

 sian, I have thought it worth while to treat the matter fully. 



As is well known, the blastoderm attains a certain size before 

 any trace of the embryo is visible, spreading by a uniform 

 growth at all points of its circumference over the yolk. 

 At Balfour's Stage A, however, the first trace of the embryo 

 appears as a slight thickening at one point of the circumfer- 

 ence of the blastoderm. This point is usually regarded as the 

 hind end of the blastoderm. This is not quite correct, for it 



