156 GENERAL ZOOLOGY 



usually do not mature at the same time, the egg in most cases is 

 fertilized by a spermatozoon from another animal. In this way 

 the occurrence of self-fertilization is reduced to a minimum. 

 When fully developed, the walls of the spermaries rupture, 

 releasing free-swimming spermatozoa into the water. When the 

 egg is mature, the wall of the ovary is partially ruptured, thus 

 exposing a portion of the free surface of the egg, through which 

 a spermatozoon may enter. The fertilized egg remains attached 

 to the wall through the early stages of development, after which 

 it is released and completes its development as an independent 

 organism. In Hydra and other invertebrate animals of a rela- 

 tively simple structure, the release of the germ cells from the 

 gonads is accomplished by the rupture of the enclosing body 

 wall, but in more complex animals, in which the gonads are 

 located internally, ducts and other accessory structures are 

 present for bringing the germ cells together or for conveying them 

 to the outside of the body, either before or after fertilization. 

 The female genital tract consists usually of a pair of oviducts, 

 one for each ovary, through which the eggs pass to the outside 

 and which, as in the frog, also provide the eggs with egg envelopes 

 or capsules. In viviparous animals a portion of the oviduct is 

 modified into a uterus in which the embryo develops until birth. 

 The so-called uterus of the frog is merely an enlarged storage 

 region in the oviduct in which eggs accumulate. In a true uterus 

 nutrition is supplied to the embryo by the walls of the uterus. 

 The vas deferens of the male corresponds to the oviduct of the 

 female. It conveys spermatozoa from the testis to the outside. 

 In the male there may also be a penis, an intromittent organ for 

 introducing sperm cells into the genital tract of the female in 

 those cases in which the eggs are fertilized internally. Internal 

 fertilization occurs in all viviparous animals and in many ovip- 

 arous ones as well. So far as development is concerned, the 

 difference between oviparous and viviparous forms consists in 

 the fact that in the former embryonic development takes place 

 after the egg is laid, and in the latter embryonic development is 

 completed before birth. 



Among segmented worms, Annulata, and also among Verte- 

 brata, there is a close relationship between the accessory parts of 

 the reproductive system and the tubules and ureter of the excre- 

 tory organs, both in embryonic development and in the adult 



