-) ZOOLOGY SECT. 



ae ovaries occur in one set of individuals — the females — and the 

 3stes in another set— the males, when the term ttnisexual or 

 "ia'cinus is employed. Very frcMjuently the male differs from the 

 3male in other respects besides the nature of the reproductive 

 lements— in size, colour, and the like ; when such differences are 

 trongly marked the animal is said to be sexualh/ dimorphic. The 

 va and sperms are usually conveyed to the exterior by canals 



r ducts the ovarian ducts or oviducts, and the testicular ducts, 



permiducts, or rasa dcferentia. In some instances the ova are 

 mpregnated after being discharged from the oviducts, and the 

 levelopment of the young takes place externally ; in other cases 

 he impregnation takes place in the oviduct, and the young 

 )ecome fully developed in the interior of a special enlargement 

 )f the oviduct termed the uterus. In the former case the animal 

 s said to be oviparm.ts, in the latter viviparous ; but there are 

 mmerous intermediate gradations between these two extremes. 



6. The Reprodxjction of Animals. 



In a limited number of groups of animal^ reproduction takes 

 ilace by means of cells corresponding to ova developed in organs 

 similar to ovaries, but witfiout impregnation by means of sperms. 

 This plienomenon is known as parthenogenesis (c/. p. 21). 



Besides the sexual process of reproduction by means of ova and 

 spermatozoa, there are in many classes of animals various asexual 

 nodes of multiplication. One of these — the process of simple 

 fission — has been alreadynoticed in connection with the reproduction 

 Df Amoeba. The formation of sjMres is an asexual mode of multi- 

 plication which occurs only in the Protozoa, and will be described 

 in the account of that group. Multiplication by ludding takes 

 place in a number of different classes of animals. In this form of 

 reproduction a process or hud (Fig. 27, hd) is given off from some 

 part of the parent animal ; this bud sooner or later assumes the 

 form of the complete animal, and maj- become detached from 

 the parent either before or after its development has been 

 completed or may remain in permanent vital connection with the 

 parent form. 



When the buds, after becoming fully developed, remain in vital 

 continuity with the parent, a sort of compound animal, consisting 

 of a greater or smaller number of connected units, is the result. 

 Such a compound organism is termed a colony, and the component 

 units are termed zooids. In some cases such a colony is produced 

 by a process which is more correctly termed incomplete fission 

 than budding. 



Alternation of generations ; heterogamy ; paedogenesis. — 

 In the life-history of a considerable number of animals, a stage in 

 which reproduction takes place by a process of budding or fission 



