Heredity and sex . Fig. 27. JS^^ as in lygaeus, and forms with an equal pair, as in oncopel-tus. Another reason is that the two sex chromosomesbehave during the synapsis stages as do the X Y chromo-somes in related species. Moreover, the experimentalevidence, of which I shall speak later, leads us to con-clude that the determination of sex is not due only to THE MECHANISM OF SEX-DETERMINATION 49 a difference in size of X and Y. The sex chromosomesmust carry a host of factors other than those that de-termine sex. Consequently it is not surprising that inmany species the sex chromosomes appear e
Heredity and sex . Fig. 27. JS^^ as in lygaeus, and forms with an equal pair, as in oncopel-tus. Another reason is that the two sex chromosomesbehave during the synapsis stages as do the X Y chromo-somes in related species. Moreover, the experimentalevidence, of which I shall speak later, leads us to con-clude that the determination of sex is not due only to THE MECHANISM OF SEX-DETERMINATION 49 a difference in size of X and Y. The sex chromosomesmust carry a host of factors other than those that de-termine sex. Consequently it is not surprising that inmany species the sex chromosomes appear equal ornearly equal in size. It is a fortunate circumstance forus that in some species there is a difference in size or. an unpaired sex chromosome; for, in consequence, weare able to trace the history of each kind of sperm inthese cases; but it is not essential to the theory thatX and Y, when present, should be visibly different. In the female of oncopeltus sixteen chromosomesoccur as in the male (Fig. 26, A). The reduced numberis eight double chromosomes {B). At one of the twopolar divisions eight chromosomes pass out, and eightremain in the egg (C). At the second division alsoeight pass out, and eight remain in the egg (D). 50 HEREDITY AND SEX I shall pass now to a fourth condition that has onlyrecently come to light. It is best shown in some of thenematode worms, for example, in the ascaris of thehorse. Here the sex chromosomes are generally at-tached to otfeg:f^rM^iQl&es. In this case, as shownby the di^Tgj^ (Fig. 27, A),HfeVe is in the male a singleX attaofim t|ipn^A)^ thej other |chromosomes. At thefirst spWfiiaiW(fyre division i*f/does not divide (C),but passe^Tdver bodily ta on^^ole, so tha
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Keywords: ., boo, bookcentury1900, bookdecade1910, booksubjectsex, bookyear1913
