FERTILIZATION 21 



ment in the uterine tube, thence passes into the uterus and becomes embedded in 

 the uterine mucosa. The time required for the passage of the ovum from the 

 uterine tube to the uterus is unknown. It probably varies in different cases and 

 may occupy a week or more. The ovum may in some cases be fertilized within 

 the uterus. Fertilization is favored by the fact that the spermatozoa swim always 

 against a current. As the ciha of the uterus and uterine tube beat downward and 

 outward the sperms are directed upward and inward. They may reach the ova- 

 rian ends of the uterine tubes within two hours of a normal coitus. 



Twin Development. — Usually but one human ovum is produced and fertilized at 

 coitus. The development of two or more embryos within the uterus is commonly due to the 

 ripening, expulsion, and subsequent fertilization of an equal number of ova. In such cases 

 ordinary or fraternal twins, triplets, and so on, of the same or opposite sex, result. Identical 

 twins, that is, those always of the same sex and strikingly similar in form and feature, are 

 regarded as arising from the daughter cells of a fertilized ovum, these having separated and 

 each having developed hke a normal ovum. Separate development of the cleavage cells can 

 be produced experimentally in many of the lower animals. The offspring of the armadillo 

 are normally produced in this manner (Patterson). 



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, maturation takes place at the expense of three potential ova, the polocytes, which de- 

 generate, but to the advantage of the single mature ovum which retains more than its share 

 of cytoplasm and nutritive yolk. 



Mendel's Law of Heredity. — Experiments show that most hereditary characters fall 

 into two opposing groups, the contrasted pairs of which 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 which are responsible for these hereditary tendencies, 

 and that paired spermatogonia! chromosomes bear one each .of these particles. Each chro- 

 mosome pair in separate germ cells may possess similar particles, both bearing black-eyed 

 tendencies or both blue-eyed tendencies, or opposing particles, bearing the one black, the 

 other blue-eyed tendencies. It is assumed that at maturation these paired particles are 

 separated along with the chromosomes, 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 combina- 

 tions with their allelomorphs from the other sex, combinations which may be favorable to 

 the offspring. 



Three combinations are possible. If the color of the eyes be taken as the hereditary 

 character, (1) 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 offspring in (1) will all have 

 black eyes, and, if interbred, their progeny will likewise inherit black eyes exclusively. 

 Similarly, the offspring in (2), and if these are interbred their progeny as well, will include 



