V ASCARIS 189 



as homologous chromosomes, and it is obvious that in each homologous 

 pair one chromosome is paternal in origin and the other maternal. 



Now when syndesis occurs it is found in cases such as I have described 

 that the two chromosomes that come together are always homologous. 

 We are justified then in defining syndesis in general as the pairing or 

 coming together of the homologous chromosomes. 



We have seen how, in Ascaris, the process of syndesis is the inaugural 

 phase of the first meiotic division and how in the course of the two 

 meiotic divisions each tetrad becomes resolved into its four constituent 

 monads. It is clear that in this latter process there take place (i) 

 separation of the two chromosomes which came temporarily together in 

 syndesis and (2) separation of the two halves into which these chromo- 

 somes were already split before syndesis took place. 



In Ascaris it is difficult to decide by actual observation what is 

 the order in which these two separations take place, but — judging by 

 the analogy of many other animals in which the matter has been worked 

 out decisively — we are justified in regarding it as probable that they take 

 place in the order named. In other words the retreat of the homologous 

 chromosomes from one another after their temporary apposition takes 

 place in all probability in the first of the two meiotic divisions. 



We have described the more conspicuous features of the processes of 

 maturation and syngamy in Ascaris but it is now necessary to say some- 

 thing about a complication in detail — difficult to observe but of great 

 importance — which has been discovered in the course of recent research. 



The kernel of the discoveries in question consists of the fact that while 

 the macrogametes are all alike, the microgametes on the other hand 

 are divisible into two distinct types, one of which produces a male and 

 the other a female zygote when it fuses with a macrogamete. We are 

 consequently brought directly into touch with one of the great problems 

 of zoological science — the determination of sex. 



It turns out that the cell of the testis which shows the two tetrad 

 chromosomes possesses in addition to these a small sex chromosome (Fig. 

 86, 1, x). This may be distinct or, as is much more frequently the case, 

 it may be unrecognizable through being fused with one of the large 

 tetrads. Now when this sex chromosome is traced through the two 

 meiotic divisions it is found that in one or other of these divisions it 

 passes bodily over to one of the two daughter cells instead of being divided 

 between them. Thus in Fig. 86, A, 2, it is seen that the sex chromosome 

 has passed bodily over into the upper cell in the first meiotic division. 

 It becomes however shared between the two daughter cells in the second 



