Heredity and sex . rprising that no one was keen enough todeduce it by purely theoretical reasoning. At leastthe general principles involved might have been de-duced, although we can see that without an intimateknowledge of the changes that take place in the germ-cells the actual mechanism could never have beenforetold. The bodies of animals and plants are composed ofmillions of protoplasm-fflled compartments that arecalled cells. In the middle of each cell there is a sphere,or nucleus, containing filaments called chromosomes(Fig. 5). At each division of a cell the wall of the nucleus isabsorb
Heredity and sex . rprising that no one was keen enough todeduce it by purely theoretical reasoning. At leastthe general principles involved might have been de-duced, although we can see that without an intimateknowledge of the changes that take place in the germ-cells the actual mechanism could never have beenforetold. The bodies of animals and plants are composed ofmillions of protoplasm-fflled compartments that arecalled cells. In the middle of each cell there is a sphere,or nucleus, containing filaments called chromosomes(Fig. 5). At each division of a cell the wall of the nucleus isabsorbed, and the thread-like chromosomes contractinto rod-shaped, or rounded bodies (Fig. 6). Eachchromosome splits lengthwise into halves; the halves 35 36 HEREDITY AND SEX are brought into relation with a spindle-shaped systemof lines, and move apart along these lines to oppositesides of the cell. The protoplasm of the cell next con-stricts to produce two daughter cells, each containinga group of daughter Fig. 19. — Fertilization and polar-body formation of Nereis. Thefour smaller figures show entrance of sperm. The extrusion of the firstpolar body is shown in lower left-hand figure and of the second polar bodyin the two large right-hand figures. The last three also show the formationof the sperm asters, which is the beginning of the first cleavage spindle inthe egg. (After F. R. LiUie.) The egg is also a cell, and in its earlier stages containsthe same number of chromosomes as do the other cells ofthe body; but after two peculiar divisions that takeplace at maturation the number of the chromosomes isreduced to half. THE MECHANISM OF SEX-DETERMINATION 37 But before this time the egg-cells divide, like all theother cells of the body. In this way a large numberof eggs is produced. After a time they cease to divideand begin to grow larger, laying up yolk and othermaterials. At this time, the chromosomes unite inpairs, so that their number seems to be reduced to halfth
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Keywords: ., boo, bookcentury1900, bookdecade1910, booksubjectsex, bookyear1913
