22 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



The Significance of Mitosis, Maturation and Fertilization. The complicated proc- 

 esses of mitosis serve the purpose of accurately dividing the chromatic substance of the 

 nucleus in such a way that the self-perpetuating chromosomes of each daughter cell may 

 be the same both quantitatively and qualitatively. This is of importance since it is 

 believed by most students of heredity that chromatin particles, or genes, in the chromo- 

 somes bear the hereditary characters, and that these are arranged in definite linear order 

 in particular chromosomes. At maturation there is a side by side union of like chromo- 

 somes, one member of each pair having come from the father, the other from the mother 

 of the preceding generation; each member, however, carries the same general set of heredi- 

 tary characters as its mate. At this stage of chromosomal conjugation there may be an 

 interchange, or 'crossing over,' of corresponding genes, resulting in new hereditary 

 combinations. The reducing division of maturation separates whole chromosomes of 

 each pair, but chance alone governs the actual assortment of paternal and maternal mem- 

 bers to the daughter cells; this mitosis obviously halves the chromosome number char- 

 acteristic for the species. The significance of the equational maturation mitosis, beyond 

 accomplishing mere cellular multiplication, is obscure. 



Fertilization initiates development and restores the original number of chromosome 

 pairs (cf. p. 20). The fertilized ovum derives its nuclear substance equally from both 

 parents, the cytoplasm and yolk almost entirely from the mother, the centrosome probably 

 from the father. 



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 

 believed that there are paired chromatic particles, or genes, which are responsible for 

 these hereditary tendencies, and that paired spermatogonial chromosomes bear one 

 each of these genes. Each chromosome pair in separate germ cells may possess similar 

 genes, both bearing black-eyed tendencies or both blue-eyed tendencies, or opposing 

 genes, bearing the one black, the other blue-eyed tendencies. It is assumed that at 

 maturation these paired genes are separated along with the chromosomes, and that one 

 only of each pair is retained in each germ cell. 



In our example, either a blue-eyed or a black-eyed tendency-bearing particle would be 

 retained. At fertilization the segregated genes of one sex may enter into new combina- 

 tions with those from the other sex. Three combinations are possible. If the color of the 

 eyes be 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 

 offspring in (i) 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 nothing but blue-eyed individuals. The first generation from 

 the cross in (3) will have black eyes solely, for black in the present example is dominant, as 

 it is termed. Such black-eyed individuals, nevertheless, possess both black- and blue- 

 eyed bearing genes their germ in cells; in the progeny resulting from the interbreeding of 

 this class the original condition is repeated pure blacks, impure blacks which hold blue 

 recessive, and pure blues will be formed in the ratio of 1:3:1 respectively. It is thus seen 

 that blue-eyed children may be born of black-eyed parents, whereas blue-eyed parents 

 can never have black-eyed offspring. Many such allelomorphic pairs of hereditary 

 characters are known. 



Cytoplasmic Inheritance. Certain eggs show distinct cytoplasmic zones which 

 cleavage later segregates into groups of cells destined to form definite organs or parts. 

 In a sense this represents a refined sort of preformation, but prelocalization is a more exact 



