44 comparative anatomy 



Developmental Potentialities 



The description of the germinal bodies up to the completion of the 

 fertilization process may very well give the impression that the motile 

 spermatozoon is the active and essentially "animal" body while the rela- 

 tively enormous egg with its burden of inert yolk is a passive and vegeta- 

 tive thing. It is true that, at first, the spermatozoon is the active and 

 aggressive body, but its activity as an independent reproductive agent is 

 very transitory, while the egg, as later events prove, is capable of becoming 

 highly dynamic and continuing so during a long period of constructive 

 development. In fact, the egg possesses this capacity quite independently 

 of any specific action of the spermatozoon. It has been proved that the 

 development of an unfertilized egg may be brought about by any one of a 

 large number of physical and chemical means, such as mere pricking with 

 a needle, or changing the osmotic pressure of the fluid external to the egg, 

 or changing the chemical constitution of the external medium. This 

 "artificial parthenogenesis" may result in the development of a perfectly 

 characteristic adult animal. Its individual peculiarities, however, are 

 only such as may have been derived from the female parent, since its 

 male "parent" may have been a mere needle prick. In normal develop- 

 ment the spermatozoon imparts the stimulus which initiates development 

 and provides for inheritance from a male parent, but the egg is fully 

 capable of producing a characteristic adult without the assistance of a 

 spermatozoon. 



Means or Exit 



The sperm is usually conveyed by way of ducts which lead from the 

 testis to the exterior, but in the cyclostome eels the sperm is discharged 

 from the testis into the body cavity and finds exit by way of abdominal 

 pores which pierce the body wall in the cloacal region. Also in some 

 Teleostei the sperm passes from the testes into the body cavity whence 

 it is discharged through genital pores. The sperm ducts are usually 

 closely associated in one way or another with the duct system of the 

 kidneys. (See Chapter ii.) 



The ova are usually liberated from the surface of the solid ovary 

 (Fig. 28) into the body cavity whence they pass into oviducts which lead 

 to the exterior. In the cyclostome eels, however, there are no oviducts, 

 and the ova, like the sperm, find exit by abdominal pores. In some bony 

 fishes (Teleostei) the ovary is hollow, the eggs are liberated into its lumen 

 and pass to the exterior by way of a duct which is an extension of the 

 wall of the ovary. In other bony fishes (the salmon and others) there 

 are no oviducts and the eggs are discharged from the body cavity through 

 abdominal pores. In viviparous vertebrates the eggs develop usually 



