SEXUAL REPRODUCTION 297 



other words, the power of regeneration in organisms is 

 most highly developed where the division of labor is com- 

 paratively slight and decreases as the cells become in- 

 creasingly specialized. 



Sexual Reproduction. — In every animal, with the 

 exception of the simplest types, the cells of the body be- 

 long to two very distinct groups. There are those con- 

 stituting the body proper, and others known as reproduc- 

 tive or germ-cells. The first form by far the greater part 

 of the individual and are more or less specialized to per- 

 form a series of activities associated with food getting, 

 avoidance of enemies, and the care of the young in many 

 cases. These functions, however, merely serve to main- 

 tain the life of the individual, and sooner or later are 

 terminated by death. 



The germ-cells, on the contrary, take no part in these 

 bodily activities, but after being nourished for a time by 

 the body are capable, under favorable conditions, of de- 

 velopment into new individuals that thus perpetuate the 

 race. Since these reproductive elements are structurally 

 unlike in the male and female, and are associated with the 

 sex of the individual, this form of multiplication is known 

 as sexual reproduction. In a subsequent paragraph it 

 will be shown that even among the unicellular animals 

 sexual reproduction occurs, and accordingly is probably 

 characteristic of every species of animal. Multiplication 

 by the sexual method depends on germ-cells or gametes, 

 and not on body-cells, while in the asexual type the new 

 individual is produced by body-cells alone. 



The Sex-Cells. — Male animals produce germ-cells 

 known as sperms or sperm-cells, while the female gives 

 rise to eggs. The sperms are developed in a compact 

 organ named the testis ; the eggs are formed in an equally 

 well-defined organ, the ovary. Each egg or sperm is a 

 single cell. The sperms differ in shape as much as the 

 species producing them. Nevertheless all agree in being 

 very small, and the majority are slender thread-like 



