CHAPTER IV. 



THE GENERAL FEATURES OF THE ELASMOBRANCH EMBRYO 

 AT SUCCESSIVE STAGES. 



No complete series of figures, representing the various stages 

 in development of an Elasmobranch Embryo, has hitherto been 

 published. With the view of supplying this deficiency Plate 

 8 has been inserted. The embryos represented in this Plate 

 form a fairly complete series, but do not all belong to a single 

 species. Figs. A, B, C, D, E, F, H, I represent embryos of 

 Pristiurus; G being an embryo of Torpedo. The remaining 

 figures, excepting K, which is a Pristiurus embryo, are embryos 

 of Scyllium canicula. The embryos A I were very accurately 

 drawn from nature by my sister, Miss A. B. Balfour. Un- 

 fortunately the exceptional beauty and clearness of the originals 

 is all but lost in the lithographs. To facilitate future description, 

 letters will be employed in the remainder of these pages to 

 signify that an embryo being described is of the same age 

 as the embryo on this Plate to which the letter used refers. 

 Thus an embryo of the same age as L will be spoken of here- 

 after as belonging to stage L. 



A. 



This figure represents a hardened blastoderm at a stage 

 when the embryo-swelling (e. s.) has become obvious, but before 

 the appearance of the medullary groove. The position of the 

 segmentation cavity is indicated by a slight swelling of the 

 blastoderm (s. c). The shape of the blastoderm, in hardened 

 specimens, is not to be- relied upon, owing to the traction which 

 the blastoderm undergoes during the process of removing the 

 yolk from the egg-shell. 



B. 



B is the view of a fresh blastoderm. The projecting part 

 of this, already mentioned as the 'embryonic rim', is indicated 





