Heredity — Genetics 711 



carried in the chromosomes of the sperm are united or fused with the 

 genes carried by the egg. In fact, fertilization is this fusion to a very 

 great extent. It will be noted that each sex cell (sperm and egg) has 

 contributed equal numbers of chromosomes to the resulting fertilized 

 cell, known as the zygote. Thus each parent has an equal opportunity 

 of contributing characteristics to the offspring, although this does not 

 mean that each parent does contribute equally, merely the opportunity 

 to do so. The number of chromosomes in the zygote is again normal. 



(k) The zygote or the first cell of the new individual is really old mate- 

 rial, but it is starting out on its journey primarily as an individual. 



The zygote divides by mitosis, giving equal halves of each chromosome 

 (also gene) to the two resulting cells. Hence, the inheritance of each 

 of these two cells is the same as that of the previous cell, or zygote. 



Mitosis of these two cells, as well as future cells, continues until there 

 is a rather solid mass of similar cells (similar as far a hereditary factors). 

 Such a mass of cells is known as the morula stage. 



(1) The cells of the morula, after a certain length of time, arrange 

 themselves in the form of a hollow sphere which is known as the hlastula 

 stage. The cells of this stage continue to divide by mitosis, thus increas- 

 ing the size of the hollow structure. Naturally, this process cannot con- 

 tinue indefinitely, or we will have an animal which will be an enormous 

 hollow ball, the wall of which will be only one cell in thickness. 



(m) By a more rapid rate of cell division or mitosis at a certain point 

 in the blastula, there is eventually an accumulation of cells at that point. 

 These numerous cells usually push inwardly or invaginate, thus forming 

 a structure which resembles a hollow ball, one side of which has been 

 partly pushed in. Such an invaginated stage is known as the gastrula 

 stage. 



Fig. 351. — Germ plasm cycle. (The normal number of chromosomes in the 

 body or somatic cells is considered to be four, or two pairs.) A, Primordial germ 

 cells of both male and female ; B, spermatogonia of male and oogonia of female ; 

 C, primary spermatocyte; D, primary oocyte; E, secondary spermatocyte; F, sec- 

 ondary oocyte; G, polar body or polocyte; H, spermatid; I, egg or ovum; /, 

 sperm; K, entrance of the sperm into the egg during the fertilization process; L, 

 zygote or first cell of the new individual in which the nuclear walls of both sperm 

 and ovum disappear, and their contents tend to fuse; note the flagellum of the 

 sperm left just outside the ovum wall; M-R, various stages in mitosis in which two 

 cells are eventually formed, each with equal chromosomes; S, four-cell stage; T, 

 morula or many-cell stage; U, blastula, or hollow-sphere stage ( half -section ) ; F, 

 early gastrula stage (half-section view) ; W, later gastrula stage in which the 

 three primary germ layers, the ectoderm, mesoderm, and entoderm, are shown; 

 the mesoderm gives rise to primordial germ cells of the next generation. 



