SPEEMATOGENESIS IN INSECTS 135 



process is merely one of repeated cell-division as in the case of 

 spermatogenesis. The three polar bodies consist almost entirely 

 of chromatin and each of course contains the same reduced 

 number of chromosomes as the ovum itself; they undergo no 

 further development, however, and finally disappear. The truth 

 of the view, now generally held, that the polar bodies are 

 merely ova which have not sufficient cytoplasm to allow of their 

 development, is demonstrated by the fact that in one of the 

 turbellarian flat-worms, according to Francotte, the exceptionally 

 large first polar body may occasionally be fertilized and actually 

 develop as far as the gastrula stage. 



The details of the process of gametogenesis vary very much 

 in different cases, but the above outline may be regarded as 

 generally applicable. 



In a large number of insects it has been found that the male 

 animal possesses an odd number of chromosomes in the somatic 

 nuclei, due to the presence of a single "accessory" chromosome 

 or " monosome," while the female possesses an even number 

 (one more than the male owing to the presence of two 

 " accessory " chromosomes). This leads to a curious complica- 

 tion in the process of spermatogenesis. The accessory chromo- 

 some (Fig. 66, X) can often be distinguished by its appearance 

 from the others, and at the time of synapsis (Fig, 66, 4) it has 

 /no mate. Hence in the reducing division the chromosomes are 

 [ separated into two unequal' groups, one of which contains the 

 accessory chromosome while the other does not (Fig. 66, 5). Two 

 kinds of spermatozoa are accordingly produced in equal numbers, 

 one kind with an odd number of chromosomes and the other with 

 an even number. * 



The matured ova, on the other hand, all have the same number 

 of chromosomes, because the accessory chromosome has a synaptic 

 mate. Fertilization of an ovum by a spermatozoon containing 

 an accessory chromosome results in the production of a female 

 animal with an even number of chromosomes in its somatic cells ; 

 fertilization by a spermatozoon which has no accessory chromo- 

 some results in the production of a male animal with an odd 

 number of chromosomes in its somatic cells, as shown in Fig. 67. 



Thus it appears that the chromosomes, at any rate in some 

 cases, have a very important influence on the determination of 

 sex, and that the latter is not, as has often been supposed, merely 

 the result of nutritional and other environmental influences upon 



