CHAPTER I. 

 THE GERM CELLS. 



The vertebrate animal body is a complex of numerous types of cells. 

 The great majority of the cells are engaged in carrying on the various activi- 

 ties of daily life. Muscle cells contract and produce motion and locomotion; 

 red blood corpuscles carry oxygen from the lungs to all parts of the body; 

 epithelial cells synthesize and secrete substances which are used in some man- 

 ner or excrete waste products; nerve cells convey impulses from one region to 

 another and thus bring distant parts into communication. All these are 

 integral parts of the body, working in harmony in response to the demands 

 put upon them. They are usually spoken of as somatic cells (soma-body) 

 because they compose the bulk of the body and are concerned in its specific 

 activities which collectively constitute the general body economy. When 

 death occurs all these cells die and disintegrate without leaving any 

 descendants. 



Within the body is another group of cells which differ in certain respects 

 from the somatic cells. They are confined to the genital or sex glands, to the 

 testis in the male and the ovary in the female. They probably play no part 

 in the general body economy; they are concerned in perpetuating the race. 

 During the life of an individual of a given generation they are discharged at 

 certain times from the glands that contain them, and under proper conditions 

 then develop into a new individual of the succeeding generation. For this 

 reason they are known as germ cells. While these cells contain the same 

 visible elements as the somatic cells, that is, nuclear and cytoplasmic com- 

 ponents, there are differences in internal organization which make these cells 

 alone capable of producing a new member of the species. Under ideal con- 

 ditions of reproduction, therefore, they do not die and disintegrate, as do the 

 somatic cells, but are carried along into and with successive generations, 

 always constituting the plasm from which new individuals arise. Each 

 sex has its own peculiar type of cell; the female carries the ovum (ovium, 

 female sex cell or germ cell), the male carries the spermatozoon (spermium, 

 sperm, male sex cell or germ cell). 



THE OVUM. 



The ovum is among the largest cells in the animal body, but varies in size 

 from a fraction of a millimeter in some of the invertebrates and in mammals 

 to several inches in the largest birds. The differences in size are due in large 



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