REPRODUCTION 45 



in an enlarged and more or less specialized region (uterus) of the oviduct, 

 but in a few of the viviparous members of the Teleostei the young develop 

 within an ovarian cavity. 



OVIPARITY, VIVIPARITY, IMPREGNATION 



The means whereby ovum and spermatozoon are brought together 

 necessarily depends on whether the animal is oviparous or viviparous; 

 also on whether or not the outer investments of the egg can be penetrated 

 by the spermatozoon. 



The great majority of fishes are oviparous and produce eggs whose 

 investing membranes can be penetrated by spermatozoa. In such cases 

 there is no copulation. The male discharges the sperm upon, or in the 

 vicinity of, the eggs after they are extruded into the water — "external 

 fertilization." Some oviparous fishes, however, produce eggs whose 

 outer covering can not be penetrated by a spermatozoon. This is true 

 of many skates and sharks. The tough thick egg-case is deposited on 

 the ovum, after it leaves the ovary, by the shell gland or oviducal gland. 

 This gland is a specialized anterior region of the oviduct. In these fishes 

 "internal fertilization" is necessary. There is copulation and the 

 spermatozoa must pass forward and effect fertilization before the egg has 

 arrived in the sheU gland, or at least before the deposition of the shell has 

 proceeded to any great extent. After such internal fertilization, the egg 

 may begin to develop and pass through the early stages before it is "laid." 

 Therefore the thing which is "laid" is not, in strict sense, an egg but an 

 early embryo. 



Some sharks and skates and a few bony fishes (the guppy, Lebistes, 

 for example) are viviparous and therefore effect internal fertihzation. 



Among Amphibia there is much diversity. In the great majority 

 of the Anura (frogs and toads) fertilization is external. At present only 

 a very few cases (certain African frogs) are known in which it is internal. 

 In the tailed amphibians (Urodela) fertilization is internal but in most 

 cases it is effected more or less indirectly by means of a spermatophore, 

 a mass of sperm agglutinated together by a secretion from the cloacal 

 glands of the male. The spermatophore may be inserted directly into 

 the female cloaca but more often is attached externally near the cloaca 

 or merely discharged and picked up later by activity of the female. In 

 caecilians (Gymnophiona) fertilization is internal. 



In all reptiles and birds the nature of the egg-shell is such as to necessi- 

 tate copulation and internal fertihzation. The substance of the shell is 

 deposited upon the egg in the posterior uterine part of the oviduct. 

 Such eggs as possess a layer of albumen acquire it from a more anterior 

 glandular region of the oviduct. Therefore fertihzation must take place 

 before the egg arrives in the uterine part of the oviduct and presumably 



