266 Preponderance of Males and Females in Fish 



brood is produced about every 14 days at 70° F. ; but one impregnation 

 will suffice for two, three, or more broods. It is thus very difficult to 

 say how many generations were involved in the whole period of altered 

 sex-ratio. The period of male preponderance would seem to be that of 

 a single generation (impregnation). During this period the prepon- 

 derance of males was noted in approximately six out of eight broods. 



We can thus distinguish (a) a period of 9 — 10 months with a pre- 

 ponderance of females (3 $ : 1 </ ) ; {h) a period of a few weeks, with a 

 lesser preponderance of males (1 ? : 1"5 or 2,/); (c) a subsequent 

 period of several years with the presumably normal sex-ratio (1 $ : 1 c/")- 



I wish hera to point out that these results are capable of interpreta- 

 tion on a hypothesis which has considerable theoretical interest, and, 

 further, is capable of verification. 



It is common knowledge that in many animals of different groups 

 sex-determination is performed by the chromosomes, one sex possessing 

 one sex-chromosome, the other two. The sex with one sex-chromosome 

 (heterogametic) may be either male or female. 



In the first case, we have the composition c^XY, %XX; while all 

 the eggs are alike, two classes of spermatozoa are produced. 



In the second case, the composition is symbolized as ^ ZZ, %ZW\ 

 there are two classes of eggs, but only one of spermatozoa. 



It might at first sight be supposed that this determination of sex 

 by the chromosomes was fixed and unmodifiable ; but there exists con- 

 siderable evidence to show that the sex-ratio may in certain cases be 

 modified by external factors. The most startling work is perhaps that 

 of R. Hertwig (1907) and Kuschakewitsch (1910), who showed in frogs 

 that, by using stale eggs, the percentage of males was greatly increased, 

 in some cultures no females appearing at all. (It may be remarked 

 that this result appears definitely not to have been due to differential 

 mortality.) 



The existence of sex-chromosomes has not actually been demon- 

 strated for frogs, although this has been done for Necturus, where the 

 male has only one X, and there are therefore two kinds of spermatozoa 

 (King, '12). If we assume, however, that the frog is like the bulk of 

 other animals which have been investigated in possessing sex-chromo- 

 somes, and further that it resembles its relative Necturus in being of 

 the %XX, (/XF type, we must suppose that the 50% of eggs which 

 were fertilized by X sperm and, being XX in composition, should 

 normally have become females, were under the conditions of the experi- 

 ment transformed into males. 



