238 Basic Structure of Vertebrates 



and eggs are liberated into its lumen and pass put by way of a duct 

 which is an extension of the wall of the ovary. 



The genital ducts are usually closely associated with the duct sys- 

 tem of the kidneys. Exceptional conditions occur in bony fishes. 



Oviparity, Viviparity, Impregnation. The means whereby ovum 

 and spermatozoon are brought together depends on whether the animal 

 is oviparous or viviparous; also on whether the outer envelopes of the 

 egg can be penetrated by a spermatozoon. 



In most oviparous fishes the eggs are impregnated after the genital 

 products have been discharged into the water ("external fertiliza- 

 tion"). But oviparous sharks and skates produce eggs whose shells 

 are impenetrable by sperm. Therefore copulation must occur and the 

 egg must be reached by the sperm before the shell is deposited. Some 

 sharks, some skates, and a few bony fishes are viviparous; copulation 

 and "internal fertilization" are therefore necessary. 



Among amphibians there is much diversity. In most frogs and 

 toads impregnation is external. In tailed amphibians (Urodela) it is 

 commonly internal, in oviparous as well as in viviparous species, and 

 in many cases is effected 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 introduced into the cloaca 

 of the female or else attached to the external surface of the female. In 

 some cases it is merely discharged, and picked up later by the female. 



Most reptiles are oviparous, but some lizards and many snakes 

 are viviparous. All birds are oviparous. But in all reptiles and birds. 

 the eggshell necessitates copulation and internal impregnation. 



Modern mammals, except the duckbill and spiny anteater, are 

 viviparous. The two exceptional animals lay eggs of reptilian sort. 

 Therefore in all mammals impregnation must be internal. 



In general, eggs which acquire such envelopes as a layer of albumen 

 or a hard shell must be impregnated while in the anterior region of the 

 oviduct and before these external coverings have been deposited. 

 Development begins immediately after fertilization. Therefore, if 

 fertilization has actually occurred, the "egg" which is "laid" by the 

 reptile or bird contains not an ovum but an embryo at an early stage 

 of development. 



Provisions for Protection, Nutrition, and Respiration. In 

 most fishes the eggs are abandoned to the hazards of the environment . 

 Some fishes, especially those of fresh water, arrange crude nests in 

 gravel, sand, or mud. Some fishes guard their eggs. In the sea horse 

 (Fig. 202) and pipefish, the male carries the developing eggs in a 

 brood-pouch on the ventral surface of the body or tail — an arrange- 

 ment suggestive of the marsupial pouch of a female kangaroo. The 



