522 LIFE : OUTLINES OF GENERAL BIOLOGY 



along three distinct lines, which are complementary, not opposed. 

 In some cases there has been a combination of two methods. 



Statistical. — Some conclusions as to the determination of the sex 

 of the offspring have been based on statistics, e.g. of the relative 

 numbers of male and female offspring in different localities, at 

 different times, with different ages of parents, and so on. 



Cytological. —Some conclusions as to the determination of the sex 

 of the offspring have been based on observations of the germ-cells 

 in particular cases. Thus it has been shown that some animals have 

 two kinds of ova, the larger developing into females. In quite a 

 number of animals there is dimorphism of spermatozoa. 



Experimental. — Some conclusions as to the determination of the 

 sex of the offspring have been based on experiment, e.g. subjecting 

 the eggs, or the embryos, or the parents to peculiar conditions of 

 nutrition, temperature, and the like, and observing whether the 

 numerical proportion of the sexes in the offspring is in any way 

 different from those obtaining in ordinary conditions; or by con- 

 trasting the results of fertilising immature and over-ripe ova; or 

 by trying particular breeding experiments in reference to what are 

 called sex-limited characters. 



CLASSIFICATION OF THE THEORIES.— There are two main 



alternatives: i. Are there two different kinds of germ-cells (male- 

 producing and female-producing), which are, in their occurrence 

 and in their development, quite unaffected by environmental 

 influence? or, 2. Do environmental influences give the germ-cell, 

 either in its early stages or during its development, a bias towards 

 becoming a male or becoming a female ? 



But a more detailed classification may be clearer and more con- 

 venient for discussion. Five theories may be distinguished. 



(a) That environmental influences, operating on the sexually 

 undetermined offspring (after fertilisation), may have at least a 

 share in determining the sex. 



(6) That the sex is undetermined until the germ-cells unite in 

 fertilisation, when it is decided by their relative condition, or by a 

 balancing of the tendencies they bear, neither sperm nor ovum being 

 necessarily decisive. 



(c) That the sex is fixed at a very early stage by the constitution 

 of the germ-cells as such, there being female- producing and male- 

 producing germ-cells, predetermined from the beginning and arising 

 independently of environmental influence. 



{d) That maleness and femaleness are Mendelian characters. 



{e) That environmental and functional influences, operating 

 through the j)arent's body, may alter the proportion of effective 

 female-producing and male-producing germ-cells. 



These five theories are not in a strict way mutually exclusive. 



