i8o CYTOLOGY chap. 



blindness and haemophilia). It is an obvious hypothesis that the reason 

 for this is that the factors of the characters in question are located in 

 the X chromosome. As the female has two X chromosomes, and as conse- 

 quently all her egg cells have an X chromosome, therefore any of them 

 are in a position to transmit this character. Since, however, half the 

 spermatozoa lack the X chromosome altogether, or possess in its place 

 the Y chromosome (which we must suppose to be inert) only half of 

 them, that is to say the X-bearing (female-producing) spermatozoa, can 

 transmit the character. Thus in Drosophila a red-eyed female can 

 transmit this character to any of her offspring, but a red-eyed male only 

 to his daughters, i.e. through the X-bearing spermatozoa. 



In other cases (Lepidoptera, Birds) it is the male which transmits 

 certain characters to all his offspring {i.e. through all gametes), and the 

 female only to her sons (i.e. through male-producing gametes only), thus 

 indicating that the chromosome formula for male and female is reversed, 

 male being XX and female XY (or X-). This again corresponds with 

 the somewhat scanty cytological observations on these two groups. 



For further information on the subject of sex-linked inheritance, 

 which furnishes one of the strongest pieces of evidence in favour of the 

 chromosomes (chromomeres) being the seat of the Mendelian factors, 

 the reader is referred to the above-mentioned works, where he will also 

 find reference to certain exceptions and the supplementary hypotheses 

 which have been put forward to account for them. 



(5) The Cytological Basis of Mutation 



If morphogenesis and heredity have their physical basis in the nucleus, 

 heritable variation must be due to changes in one or more of the chromo- 

 somes, or, rather, of their constituents. This of course is, in the present 

 state of our knowledge, impossible to prove directly, but certain relative 

 evidence can be obtained from the study of the origin of variation, or 

 mutation. 



If a hereditary factor undergoes a mutational change, whether in 

 gametogenesis or at some other time, the first individual to possess this 

 altered factor in its diploid nuclei will presumably most often possess 

 one chromosome containing the factor in its new state, and its homologue 

 possessing it in its old state. A moment's consideration will make this 

 clear. If the variation has taken place in gametogenesis, the mutated 

 gamete will almost necessarily have to fertiUze a non-mutated gamete 

 from another organism, for its chances of meeting another gamete which 

 has undergone a similar mutation will in most cases be negligible, and 

 hence the first individual to possess the new factor will be heterozygous 

 between it and the old form. If on the other hand the mutation takes 



