DETERMINATION OF SEX 3 1 



Usually but one human ovum is produced and fertilized at coitus. The de- 

 velopment of two or more embryos within the uterus may be due to the ripening 

 and expulsion of an equal number of ova at ovulation, these being fertilized later. 

 Identical twins are regarded as arising from the daughter cells of a fertilized ovum, 

 these cells having separated, and each having developed like a normal ovum. 



The Significance of Mitosis, Maturation and Fertilization. — It is assumed by 

 students of heredity that the chromatic particles of the nucleus bear the hereditary qualities 

 of the cell. During the course of development these particles are probably distributed to the 

 various cells in a definite way by the process of mitosis. The process of fertilization would 

 double the number of hereditary qualities and they would be multiplied indefinitely were it not 

 for maturation. At maturation not only is the number of chromosomes halved, but it is 

 assumed also that the number of hereditary qualities is reduced by half. In the case of the 

 ovum, this takes place at the expense of three potential ova, the polocytes, which degenerate, 

 but is to the advantage of the single mature ovum which retains m^re than its share of cytoplasm 

 and nutritive yolk. 



Mendel's Law of Heredity. — Experiments show that all hereditary characters fall into two 

 opposing groups, which alternate with each other and are termed allelomorphs. As an example, 

 we may take the hereditary tendencies for black and blue eyes. It is supposed that there are 

 paired chromatic particles in the germ cells which bear these hereditary tendencies. Each 

 pair may be composed of similar particles, both bearing black-eyed tendencies or both blue- 

 eyed tendencies, or opposing particles may bear the one black, the other blue-eyed tendencies. 

 It is assumed that at maturation these paired particles are separated, and that one only of each 

 pair is retained in each germ cell, in order that new and favorable combinations may be formed 

 at fertilization. In our example, either a blue-eyed or a black-eyed tendency bearing particle 

 would be retained. At fertilization the segregated tendency-bearing particles of one sex may 

 enter into new combinations with the allelomorphs of the other sex, combinations which may 

 be favorable to the offspring. Three combinations may be possible. If the color of the eyes 

 is taken as the hereditary character, (i) two "black" germ cells may unite; (2) two "blue" 

 germ cells may unite; (3) a "black" germ cell may unite with a "blue" germ cell. The result- 

 ing individual will be in (1) black-eyed; in (2) blue-eyed; in (3) either black-eyed or blue-eyed, 

 according to whether one or the other tendency predominated. Were the black-eyed tendency 

 in (3) predominant and the resulting individual black-eyed, there would still be blue-eyed 

 bearing chromatin particles in his or her germ cells. In the next generation these recessive 

 blue-eyed qualities may unite with similar qualities of another black-eyed individual. The 

 offspring would be blue-eyed, though both the parents were black-eyed. 



DETERMINATION OF SEX 

 The assumption that the chromosomes are the carriers of hereditary ten- 

 dencies is borne out by the observations of cytologists on the germ cells of insects 

 and some vertebrates. It has been shown that in some forms the nucleus of the 

 spermatogonia contain 23 chromosomes, while those of the oogonia contain 24. 

 When maturation and reduction of the chromosomes take place, half of the sper- 

 matids contain 1 2 chromosomes, the other half only eleven, while all the oocytes 



