72 MANUAL OF ZOOLOGY sect. 



The organs of sexual reproduction are the gonads, in 

 which male and female cells or sperms and ova are 

 produced, with the gonoducts or canals by which these 

 cells reach the exterior. The gonads in which male cells 

 or sperms are produced are called testes, and their ducts 

 are the sperm-ducts. The gonads in which female cells 

 or ova are formed are called ovaries, and their ducts 

 oviducts. Sometimes testes and ovaries occur in distinct 

 male and female individuals, when the animal is said to be 

 unisexual, or to have the sexes distinct. In other cases 

 both ovaries and testes occur in the same individual, when 

 the animal is said to be hermaphrodite, or to have the sexes 

 united. In some instances the same gonad produces both 

 sperms and ova — assuming the character of a hermaphrodite 

 gonad or ovo-testis. 



In many animals the ova are fertilised by the sperms 

 after they have passed out from the body, and the develop- 

 ment takes place externally — a condition which is known 

 as oviparity. But in others the ova are fertilised while still 

 in the ovary or oviduct of the parent, and the development 

 may take place in the oviduct, usually in a special dilated 

 part of the latter — the uterus — so that the young only 

 escape to the exterior after they have attained a compara- 

 tively advanced stage of their development — when the 

 animal is said to be viviparous. 



Besides the sexual process of reproduction by means 

 of ova and sperms, there are in many classes of animals 

 various asexual modes of multiplication. One of these — 

 the process of simple fission — has been already noticed in 

 connection with the reproduction of Protozoa. The forma- 

 tion of spores is an asexual mode of multiplication which 

 occurs only in the Protozoa, and has been described in the 

 account of that group. Multiplication by budding takes 



