45O HISTOLOGY 



number of chromosomes (35) in the spermatozoon; (2) an even number 

 of chromosomes (36) in the follicle cells of the ovary (it has been proved 

 in various forms of insects that the number of somatic chromosomes is 

 the same as that of the presynaptic germ cells); (3) the appearance of 

 a chromatin nucleolus in the spermatogonial cells which persists during 

 the last mitosis preceding the spermatocyte phase and by its staining 

 reaction and behavior during maturation proves itself a true chromosome 

 (accessory chromosome); (4) the absence of a mate with which the acces- 

 sory may pair during synapsis; (5) failure of the accessory to divide in 

 the first maturation mitosis ; (6) a resulting dimorphism of spermatozoa, 

 consisting in the presence (18 chromosomes) and absence (17 chromo- 

 somes) of the accessory; (7) a reduction in the number of chromosomes, 

 during the maturation of the egg, to half the somatic number (18 chro- 

 mosomes). 



From the above facts it follows that, if a mature egg possessing 18 

 chromosomes is fertilized by a spermatozoon with 18 chromosomes, an 

 organism results which has 36 somatic chromosomes, and this is known 

 by observation to be a female. Again if such an egg be fertilized by 

 a spermatozoon with 17 chromosomes, an organism with 35 chromosomes 

 results and this is known to be a male. The presence of an additional 

 chromosome (the accessory) thus distinguishes the female from the male 

 cell from the chromosome standpoint ; hence the accessory chromosome 

 appears to have some connection with the kind of sex that is to be pro- 

 duced. The final test of the theory that the accessory chromosome is 

 a sex-determinant lies in direct experiment with that element in fertili- 

 zation, the difficulties of which have thus far remained insuperable. 

 Meanwhile, however far the theory may accord with fact, it remains a 

 question, as Bateson has suggested, whether the accessory body may not 

 be merely associated with the cause of sex; or, as Wilson suggests, the 

 morphological expression of a hidden physiological cause. The most 

 that may perhaps be claimed for the accessory, in relation to sex pro- 

 duction, is that it represents sex characters, and this is further based on 

 the assumption that the chromosomes really are the vehicles that carry 

 the hereditary characters. 



As alternative theories, Wilson suggests: (i) that the heterochro- 

 mosomes may merely transmit sex characters, and that sex itself is 

 determined by protoplasmic conditions external to the chromosomes; 

 (2) that the accessory may be a sex determinant only by virtue of a dif- 

 ference in activity or amount of chromatin. Paulmier and Montgomery 

 believe that the accessory is a degenerating chromosome and that its 

 presence represents a stage in the evolution of a species from a higher to 

 a lower chromosome number. They further believe, with Wilson, that 

 it represents the persisting larger member of a pair of idiochromosomes, 



