678 JOHN BEARD, 



With the like preservation a Scyllium-embryo of 32 — '6'6 mm in 

 ray collection always represents in the degree of development of its 

 organs a Raja batis of about 65 — 70 mm. Approximately, therefore, 

 embryos of the latter are double the length of corresponding ones of 

 the former, and this rough rule applies to all periods of the devel- 

 opment. It is possible, that Balfour's embryos were smaller than 

 mine of the above and of other periods. The latter were obtained in 

 Wales, and there is a strong suspicion in my mind, which as yet it 

 has not been possible to test on actual specimens, that the Welsh 

 form is a variety distinct from the Naples one, and, moreover, one 

 whose embryos are rather larger than those of the latter at cor- 

 responding periods of the development. 



Reverting to Balfour's work, fig. 1, tab. 17, cannot represent 

 "primitive ova": certainly, I myself have never seen 22, or anything 

 like that number of, primary germ-cells in a single section of one 

 ovarian ridge of Scyllium. Moreover, my experiences in the skate and 

 in Scyllium lead to the conclusion, that the "primitive ova" begin to 

 divide and to form secondary germ-cells prior to Balfour's stage P (in 

 embryos of 25—26 mm). From their number and sizes, and from the 

 phase of the embryo, the conclusion seems warranted, that the cells 

 of the above figure are all secondary germ-cells. 



It is, of course, quite natural, that the secondary germ-cells, i. e. 

 "the permanent ova" of Balfour, should ultimately be larger than 

 the primary ones; for on them falls the lot of becoming the sexual 

 products. But that the newly formed secondary germ-cells are larger 

 than the primary ones is not in accordance with the facts; and, as 

 Balfour neither counted, nor at that time could count, the germ-cells 

 at any period, nor made even approximate estimates of their number 

 and distribution, the period of the first formation of secondary germ- 

 cells was overlooked ; or, at any rate, referred to an epoch, later than 

 that, at which it really happens. 



On p. 396 he writes: "the passive condition of the primitive ova 

 becomes suddenly broken during stage Q, and is succeeded by a 

 period of remarkable changes". That is, in the text he records the 

 change as beginning at a period even later than that, at which he 

 really depicts it in fig. 1, tab. 17. 



After all the main point is, that Balfour distinguished between 

 "primitive ova" and "permanent ova", also noting the long period of 

 quiescence before the latter begin to be formed. 



