Heredity and sex . jy^ Fig. 23. ^^ number of chromosomes as in protenor, but by theoccurrence of a pair of different-sized chromosomes. The body cells of the male have twelve ordinarychromosomes and two sex chromosomes — one larger,X, than the other, Y (Fig. 23, A). After synapsis there are six double chromosomes andthe two sex chromosomes, called X and Y (Fig. 23, D). THE MECHANISM OF SEX-DETERMINATION 45 At the first spermatocyte division all the chromosomesdivide (C). The two resulting cells have eight chro-mosomes, including X and Y. At the second division(D) the double chromosomes again d
Heredity and sex . jy^ Fig. 23. ^^ number of chromosomes as in protenor, but by theoccurrence of a pair of different-sized chromosomes. The body cells of the male have twelve ordinarychromosomes and two sex chromosomes — one larger,X, than the other, Y (Fig. 23, A). After synapsis there are six double chromosomes andthe two sex chromosomes, called X and Y (Fig. 23, D). THE MECHANISM OF SEX-DETERMINATION 45 At the first spermatocyte division all the chromosomesdivide (C). The two resulting cells have eight chro-mosomes, including X and Y. At the second division(D) the double chromosomes again divide, but X and Ydo not divide. They approach and touch each other,and are carried into the spindle, where they separatefrom each other when the other ordinary chromosomes jLygaeu-f ^ •••V. A »3^ 3. divide. Consequently there are formed two kinds ofspermatozoa — one containing X and the other Y(Fig. 23, E). In the body cells and early germ-tract of the femaleof lygseus (Fig. 24, A), there are twelve ordinarychromosomes and two sex chromosomes, X and reduction there are seven double chromosomes,the two Xs having united when the other chromosomes 46 HEREDITY AND SEX united (B). Two divisions take place {C, D), when thetwo polar bodies are formed, leaving seven chromosomesin the egg (E). Each egg contains as a result only oneX chromosome. Any egg of lygseus fertilized by a sperm carrying anX chromosome produces a female that contains two OncopettcLS o B
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Keywords: ., boo, bookcentury1900, bookdecade1910, booksubjectsex, bookyear1913
