File:Heredity and sex (1913) (14780550485).jpg

Identifier: hereditysex1913morg (find matches)
Title: Heredity and sex
Year: 1913 (1910s)
Authors: Morgan, Thomas Hunt, 1866-1945
Subjects: Heredity Sex Heredity
Publisher: New York, Columbia University Press
Contributing Library: Francis A. Countway Library of Medicine
Digitizing Sponsor: Open Knowledge Commons and Harvard Medical School

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NISM OF SEX-DETERMINATION 53 zoa. The large cell divides again, all of the chromo-somes dividing. Two functional spermatozoa areproduced, each carrying one sex chromosome. Thesespermatozoa correspond to the female-producing sper-matozoa of other insects. In the sexual female there is an even number of chro- f. • • - >-K FiG. 29a. — 20 and 21, oogonia (equatorial plate) ; 22, growth period;23, before fertilization; 24-25, entrance of sperm; 26-31, prophases offirst division; 32-33, formation of first polar body; 34-36, extrusion ofsame and formation of second polar body; 3>1, two pronuclei; 38-41, unionof pronuclei; 42-45, cleavage. (After Mulsow.) mosomes — one more than in the male. They unitein pairs. When the two polar bodies of the sexualegg are formed, all the chromosomes divide twice, sothat each egg is left with one sex chromosome. It is now evident why only females are producedafter fertilization. The female-producing sperm aloneis functional. 54 HEREDITY AXD SEX
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Fig. 30. — Diagram of chromosomes in Phylloxera carycEcauUs. Topline, somatic cell of female with 6 chromosomes and somatic cell of male■v\-ith 5 chromosomes. Second line, stages in first spermatocyte divisionproducing a rudimentary cell (belowj with two chromosomes. Third line,second spermatocyte di\ision into two equal cells. Fourth line, sexualegg (3 chromosomes) and two polar bodies ; and two functional, female-producing sperm with three chromosomes each. THE MECHAXIS)M OF SEX-DETERMINATIOX 55 THE EXPERIMEXTAL EVIDEXCE The experimental evidence, indicating that there isan internal mechanism for sex determination, is derivedfrom two som^ces — from experimental embryology, andfrom a study of the heredity of sex-Hnked characters. The evidence from embr^^ology shows that the chro-mosomes are the bearers of materials essential for theproduction of characters. The evidence from hered-ity shows that certain characters follow the sexchromosomes. It has long been taught that the here

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