Heredity and sex . Fig. 25. :e^ Xs or XX. Any egg fertilized by a sperni containinga Y chromosome produces a male that contains oneX and one Y, or XY. Another insect, Oncopeltus fasciatus, represents athird type in which the chromosome groups in themale and in the female are numerically alike and alikeas to visible size relations. THE MECHANISM OF SEX-DETERMINATION 47 In the body cells of the male there are sixteen chro-mosomes (Fig. 25, A). After reduction there are ninechromosomes — seven in a ring and two in the middle(B). The seven are the fused pairs or double chro-mosomes; the two in the
Heredity and sex . Fig. 25. :e^ Xs or XX. Any egg fertilized by a sperni containinga Y chromosome produces a male that contains oneX and one Y, or XY. Another insect, Oncopeltus fasciatus, represents athird type in which the chromosome groups in themale and in the female are numerically alike and alikeas to visible size relations. THE MECHANISM OF SEX-DETERMINATION 47 In the body cells of the male there are sixteen chro-mosomes (Fig. 25, A). After reduction there are ninechromosomes — seven in a ring and two in the middle(B). The seven are the fused pairs or double chro-mosomes; the two in the middle are the sex chromo-somes that have not Z> Fig. 26. The evidence for this interpretation is circumstan-tial but sufficient. At the first reduction division all nine chromosomesdivide (C). Just before the second division the twocentral chromosomes come together and remain incontact (DD). All the double chromosomes thendivide, while the two sex chromosomes simply sepa-rate from each other, so that there are eight chromo-somes at each pole (DE). 48 HEREDITY AND SEX In this case all of the spermatozoa (EE^) containeight chromosomes. There is no visible differencebetween them. Nevertheless, there is reason for be-lieving that here also there are two kinds of principal reason is that there are all connectingstages between forms in which there is an unequal pair, ^scares S i\} •%~ sPs
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Keywords: ., boo, bookcentury1900, bookdecade1910, booksubjectsex, bookyear1913
