43o HEREDITY AND SEX 



ciated with the fact that one half of the spermatozoa have " an 

 accessory chromosome," absent in the other half. Apart from size- 

 dimorphism, the occurrence of an accessory chromosome in half of 

 the spermatozoa has been observed in numerous and varied types 

 of animals. There is interesting indirect evidence that the ova fer- 

 tilised by spermatozoa with the accessory chromosome develop into 

 females, while those fertilised by spermatozoa without the accessory 

 chromosome develop into males. 



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 particular 

 conditions of nutrition, temperature, and the like, and observing 

 whether the relative numbers of the sexes in the offspring are in 

 any way different from those obtaining in ordinary conditions ; 

 or by contrasting the results of fertilising immature and over- 

 ripe ova ; or by trying particular breeding experiments in 

 reference to what are called sex-limited characters. 



4. Classification of the Theories 



There are two main alternatives : (i) Are there two kinds 

 of germ-cells (male-producing and female-producing), which are, 

 in their occurrence and in their development, quite unaffected 

 by environmental influence ? or, (ii) Do environmental influences 

 give the germ-cell, either in its early stages or during its develop- 

 ment, a bias towards male-production or female-production ? 



But a more detailed classification may be clearer and more 

 convenient for discussion. Five theories may be distinguished. 



(a) That environmental influences, operating on the sexually 

 undetermined offspring (after fertilisation), may at least have 

 a share in determining the sex. 



(b) 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 constitu- 



