iv THE SEX CHROMOSOMES 115 



of course contain 10 + XX chromosomes. From now onwards, however, 

 the sex chromosomes of the two kinds of cells behave differently. 



In the oogenesis there is nothing noteworthy, all the chromosomes 

 behaving alike, and all the mature eggs possessing six (5 + X) chromo- 

 somes. 



In those cells which are going to give rise to spermatozoa, however, 

 and which may therefore now be called primary spermatocytes, one of 

 the X chromosomes undergoes a change which may perhaps legitimately 

 be expressed by saying that it turns into a Y chromosome. This 

 chromosome condenses out of the diffuse stage sooner than any of the 

 others (Fig. 52, E), its mate, the remaining X chromosome, following 

 soon after (Fig. 52, F). From now onwards the meiotic phase proceeds 

 in the typical manner for an animal with an XY pair, the second division 

 being the differential one so far as they are concerned. 



Two classes of spermatids are formed, one with 5 + X, the other with 

 5 + Y chromosomes. Of each pair of spermatids one (of the formula 

 5 + X) develops in the usual way into a spermatozoon containing six 

 chromosomes. In the other, however, the Y chromosome fails to enter 

 into the nucleus, but remains outside in the cytoplasm, to be ultimately 

 cast off with the excess cytoplasm (cytophore) when the ripe spermatozoon 

 is freed. Thus two classes of spermatozoa are formed, one with 5 and one 

 with 5 + X chromosomes. 



It should be noted that Boveri found the process less regular than 

 this, but with the same end result namely, the same two classes of 

 spermatozoa, in which five and six chromosomes can be counted 

 respectively. 



The conjugation of these spermatozoa with the ova brings us back 

 to our starting-point the bisexual generation, the males of which have 

 eleven and the females twelve chromosomes. 



(b) Aphids and their Allies. The eggs laid in autumn are fertilized 

 and in the spring hatch into females, which reproduce parthenogenetically 

 (with one maturation division and no reduction of chromosomes). After 

 a lapse of one or more parthenogenetic generations, sexual forms (i.e. 

 males and sexual females) are produced ; copulation takes place, fertilized 

 eggs result, and the life-cycle is complete. 



Here the double problem arises : 



(1) How is it that all fertilized eggs produce females only ? 



(2) What determines the sex of the individual developed from an 

 unfertilized egg, and which is sometimes male and sometimes female ? 



The answer to the first problem is very clearly given by von Baehr 

 (1912) for Aphis saliceti. In this species the diploid formula for the female 

 is 4 + XX and for the male 4 + X. Examination of spermatogenesis 

 (Fig. 53) shows that the X chromosome does not divide, but passes 



