190 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 germinal cells to undergo 

 parthenogenetic development, or to develop without 

 fertilization, also quickly disappears, not being observed 

 among animals higher than the arthropods. 



Parthenogenetic development seems to take place only in 

 female germinal cells in which there is no reduction of chro- 

 mosomes (see below) and in which there is no conjugation 

 with male germinal cells. It must not be confused with 

 hermaphroditism. It occurs notably in certain rotifers, the 

 males of which are not known, if they exist at all; in cer- 

 tain Branchiopods and Astracods, in Daphnids during 

 summer; among Aphides or plant-lice, and among certain 

 bees (social bees). It is difficult to explain, but Owen, as 

 early as 1849, suggested that "not all of the progeny of 

 the primarily impregnated germ-cell are required for the 

 formation of the body in all animals; certain of the 

 derivative germ-cells may remain unchanged and become 

 included in that body which has been composed of their 

 metamorphosed and diversely combined or confluent 

 brethren; so included, any derivative germ-cell, or the 

 nucleus of such, may begin and repeat the same proc- 

 esses of growth by imbibition, and of propagation by 

 spontaneous fission, as those to which itself owed.. its 

 origin." It is to Owen that we owe the word "partheno- 

 genesis." 



