334 ! ANIMAL BIOLOGY 



lice in which the females may reproduce by parthenogene- 

 sis for several generations, especially during the summer. 

 After a time, however, different kinds of eggs are produced 

 which require fertilization before they develop. It has 

 been found by Loeb that in some animals eggs which nor- 

 mally require fertilization before developing, maybe stimu- 

 lated artificially by chemicals and other agencies so that 

 they develop without fertilization into apparently normal 

 embryos. In almost all forms in which parthenogenesis 

 occurs, reproduction by means of fertilized eggs occurs also 

 after one or more parthenogenetic generations. 



While in most animals sperm and egg cells are produced 

 by separate male and female individuals, there are many 

 animals in which both kinds of sex cells are borne in the same 

 body. Such animals are called hermaphrodites. Most 

 flat worms, earthworms, leeches, land snails and tunicates 

 are hermaphrodites, and there are occasional hermaph- 

 roditic species in many other groups of animals, and 

 exceptional hermaphrodites which arise as " sports" 

 or monstrosities in species with normally separate sexes. 

 It is a curious fact that in hermaphroditic animals the 

 eggs are very rarely fertilized by sperms from the same 

 individual, but instead there is cross fertilization, as we 

 have seen in our account of the earthworm. In sexual 

 reproduction in general there is a mingling of germinal 

 material derived from two separate individuals. 



After the union of the egg and sperm cell there begins 

 the process of embryonic development which results 

 in the formation of a new individual. This process, 

 which is one of great complexity, forms the subject 

 matter of the science of embryology, a subject which can 

 be treated but very briefly in an elementary book. The 

 egg in all animals consists of a single cell. Usually this 

 is of minute size, but in birds and many reptiles and in 



