560 THE HUMAN BODY 



the continuance of the race is specially provided for by setting 

 apart certain cells which shall have this one property cells whose 

 duty is to the species and not to any one representative of it an 

 essentially altruistic element in the otherwise egoistic whole. 



Germ-Cells Compared with Tissue-Cells. Those cells which 

 are set apart for the maintenance of the race are called germ-cells 

 to distinguish them from the cells which make up the Body gen- 

 erally and which are designated as somatic cells. Each individual 

 is derived from a single germ-cell, as noted in an earlier chapter 

 (p. 29), by a process of cell multiplication and cell differentiation. 

 The controlling factor in these processes was shown to be the 

 chromatin network of the nucleus, made up of a definite number 

 of chromosomes. An important feature of the difference between 

 germ-cells and somatic cells is that the former contain a much 

 larger amount of chromatin material than do the latter. At the 

 beginning of cell multiplication in the ovum (p. 25) the daughter 

 cells are alike in chromatin content, but a stage is soon 

 reached, very early in some forms, in which many of the daughter 

 cells discharge a part of their chromatin. The part eliminated 

 passes out of the nucleus into the mass of the cell and is dissolved. 

 Those cells in which this occurs are destined to develop into the 

 somatic or general tissues. Those that retain their full comple- 

 ment of chromatin become the germ-cells of the adult organism. 



Sexual Reproduction. In some cases, especially among insects, 

 the specialized reproductive cells can develop, each for itself, 

 under suitable conditions, and give rise to new individuals; such 

 a mode of reproduction is called parthenogenesis: but in the major- 

 ity of cases, and always in the higher animals, this is not so; 

 the fusion of two cells, or of products of two cells, is a necessary 

 preliminary to development. Commonly the coalescing cells 

 differ considerably in size and form, and one takes a more direct 

 share in the developmental processes; this is the egg-cell or ovum; 

 the other is the sperm-cell or spermatozoon. The fusion of the 

 two is known as fertilization. Animals producing both ova and 

 spermatozoa are hermaphrodite; those bearing ova only, female; 

 and those spermatozoa only, male: hermaphroditism is not found 

 in Vertebrates, except in rare and doubtful cases of monstrosity. 



Maturation of the Germ-Cells. In the germinal tissues of the 

 sexually mature individual cell multiplication goes on actively 



