56 



It must not be forgotten that the numbers 256 and 512 

 are not absolutely correct, for, in addition to the one used in 

 the formation of the embryo containing them, a certain 

 number degenerate. 



Although it is demonstrated that the primary germ-cell 

 giving rise to the male skate is one of the definite number of 

 256, and the female from one of 512, it would be unsafe to 

 generalize and assume that the number of germ-cells from 

 which arise females is always double the number giving rise to 

 males, for in Scyllium canicida the number giving rise to 

 males and females is the same, namely, 128. 



Moreover, the number of primary germ-cells varies with 

 the genera and species. Without definite knowledge as to 

 the sex the following numbers of primary germ-cells are 

 recorded : — Raiia esculenta, 8 ; Petromyion Planeri, 32 ; 

 Acaiithias vulgaris (64 & 128?) ; Pristinrus melanosto)iius, 128; 

 Cyclops, 2 ; Ascaris, 2 ; Cecydomyia, 4 ; and Chirotiomns, 4. 



Time will not permit me to pursue this interesting subject 

 further, it must suffice to indicate in a few words from Beard, 

 where, in the course of development, sex is determined. 



He writes: — "From the facts concerning the number of 

 " primary germ-cells, and from certain other factors, . . . . 

 " it may be concluded that sex is actually differentiated 

 " and decided during oogenesis. The facts point to the last 

 " division of the oogonia, and the formation of oocytes, as the 

 " particular epoch at which this happens. That is, the oocytes 

 " are differentiated into two categories, destined to become male 

 " and female eggs respectively."" 



Beard is of opinion that the initiation of the process 

 with the change from primary to secondary germ-cells, accounts 

 for the inability of the secondary germ-ceils to undergo 

 independent development. 



» John Heard. ' The Determination of Sex, etc.," p. 713. 



