188 BIOLOGY: GENERAL AND MEDICAL 



to reach one another for reproductive purposes. And 

 we see among the hermaphrodite animals the same 

 repugnance to self-fertilization where it is not made 

 essential by the peculiar conditions of life. Thus of 

 hermaphroditic animals we find self-fertilization prac- 

 tised by the parasitic worms whose existence is so 

 precarious that it is exceptional to find more than one in 

 the same host, but not among the earthworms or snails 

 that are free and independent. 



The higher we ascend in the animal kingdom, the 

 more emphatic is the demand for conjugation of gametes 

 from separate individuals of opposite sexes. Not only 

 does sexual differentiation take place low down in the 

 scale of life, but the ability of the gametes of sexual 

 cells to undergo parthenogenetic development, or to 

 develop without fertilization, also quickly disappears, 

 not being observed among animals higher than the 

 arthropods. Development by gemmation also disap- 

 pears very early, so that there is nothing in the higher 

 animal organisms that is in the least degree comparable 

 to the multiplication by budding so prevalent among 

 the higher plants. Reproduction among animals, there- 

 fore, soon narrows itself down to the formation of gam- 

 etes, produced by sexually different indviduals, the con- 

 jugation of these gametes and the formation of a zygote 

 or fertilized cell which grows into an embryo, which after 

 certain metamorphoses develops into a sexual individual 

 resembling one or the other parent. 



There is an early differentiation of the cells of animals 

 into those known as somatiCj of which the body proper 

 consists, and those known as germinal whose office is 

 solely reproductive. The latter are contained in the 

 gonads or sexual organs until the organism to which 

 they belong becomes sexually mature, when they them- 

 selves undergo certain essential changes preparative to 

 the fertilization that initiates the reproductive process. 



In vegetables it cannot be shown that the germinal 

 cells differ essentially from the somatic cells. They 



