REPRODUCTION AND SEX 527 



mice, and found that the proportions of the sexes in mice were 

 unaifected by the age of the parents, by apparent vigour, by con- 

 sanguineous union, by frequency of births, or by any kind of 

 nutritive change. 



(c) THIRD THEORY— That the sex is fixed at a very early stage 

 by the constitution of the germ-cells as such, there being female-pro- 

 ducing and male-producing germ-cells, predetermined from the 

 beginning and arising independently of environmental influence. 



On this view there are two kinds of germ-cells, constitutionally 

 predetermined to be female-producers or male-producers. This 

 implies that the sex is determined before fertilisation, thus excluding 

 the second theory. It also implies that the influence of the environ- 

 ment is negligible after the germ-cells have been estabHshed, and 

 a fortiori after development has begun, thus excluding the first 

 theory. 



Two Kinds of Ova. — What evidence is there of two kinds of 

 ova — one kind constitutionally predestined to develop into males, 

 the other kind constitutionally predestined to develop into females ? 



Some animals normally produce two sizes of ova. Thus in 

 Phylloxera among Insects, and Hydatina senta among Rotifers, 

 there are large eggs which develop into females, and small ones 

 which develop into males. As both develop without fertilisation, the 

 problem is not complicated by the influence of the sperm. 



In Dinophilus apatris, according to Von Malsen, and in a mite, 

 Pediculopsis, according to Renter, where fertilisation occurs as 

 usual, there are large ova which develop into females and small 

 ova which develop into males. In Dinophilus, the ovum which 

 becomes a male is only about one-tenth of the size of that which 

 becomes a female, and the male himself is a degenerate pigmy! We 

 must not hastily assume that it is the size that determines the sex, 

 since it may be that the constitutional predisposition to one sex or 

 the other determines the size. 



In some of the higher Pteridophytes there are two kinds of 

 spores, micro- and macro-spores, which produce respectively male 

 and female prothalli. A similar predestination, not marked by 

 visible differences, has been proved by Blakeslee in both zygotes 

 and spores of various species of Fungi, and it has also been demon- 

 strated in liverworts and mosses. In the studies of the Marchals 

 on dioecious mosses, isolation cultures prove that the spores, 

 though similar in appearance, are individually predestined, 

 indeed fixedly predetermined, to develop into male or female 

 gametophytes. 



The view that there are two kinds of ova, determined ab initio 

 as male-producers and female-producers, had a vigorous supporter 

 in Beard, who found evidence in the skate. He maintained that the 



