BY WILLIAM A. HASWELL. 9T 



indebted to my demonstrator, Mr. J. P. Hill, not only for help in 

 obtaining the specimens, but also for presei'ving many of the 

 blastoderms, for making measurements and indicating approxi- 

 mately the stage which each had reached, and for sectioning many 

 of them. 



Cestracion (Heterodontus) is a genus which is almost unique 

 among the Elasmobranchs in having been represented by near 

 relatives as far back as the Carboniferous period. Although the 

 adult structure of the rnembers of the family had even at that 

 early period become highly specialised and widely divergent frora 

 the main line of Elasmobranch evolution, the hope is not an 

 unreasonably' sanguine one that the embryonic development of a 

 type so ancient might exhibit some important primitive features. 

 With regard to the stages now described, however, any expecta- 

 tions of the kind cannot be said to have been fulfilled; and what 

 impresses one most in the results is the extraordinary persistency 

 of certain characteristics which are not known to have any vital 

 significance. There can be little doubt, for example, that the 

 " orange spot " which forms such a striking feature of the egg of 

 an Elasmobranch in its early stages, has been handed down with 

 little change from Palifiozoic times. 



The l)lastoderm occupies a constant position in the egg. It is 

 always situated much nearer the Ijroader than the narrower end 

 of the egg-shell. The extremity of the blastoderm destined to 

 become posterior is always directed away from the broader end 

 of the egg-shell. Balfour* states that in Pristiiii-us the blasto- 

 derm is similarly constant in its position near the rounded end of 

 the egg, while in Scyllium it is always near the narrow end to 

 which the shorter pair of filaments is attached. 



The blastoderm, in its earlier stages, appears to the naked eye, 

 as in other Elasmobranchs,f as a circular reddish orange spot, 

 around which is a narrow light yellow band. When this orange spot 

 has attained a diameter of about 2 mm. it assumes an oval shape, its 



* Balfour, Development of Elasmobranch Fishes, p. 10. 

 t L.C. p. 10. 



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