CHAPTER IV. 



The General Features of the Elasmobranch Embryo 



at successive stages. 



No complete series of figures, representing the variuus 

 stages in development of an Elasmobranch Embryo, has 

 hitherto been published. With the view of supplying this 

 deficiency Plates vi. and vii. have been inserted. The 

 embryos represented in these two Plates form a fairly com- 

 plete series, but do not all belong to a single species. Those 

 on PI. VI., with the exception of G, are embryos of Pristi- 

 urus ; G being an embryo of Torpedo. Those on PL Vll., 

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

 Scyllium canicula. All the embryos on PL vil. were very 

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

 Unfortunately 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 these Plates to which the 

 letter used refers. Thus an embryo of the same age as L will 

 be spoken of hereafter 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 posi- 

 tion of the segmentation cavity is indicated by a slight swell- 

 ing of the blastoderm (5. 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 



