REPRODUCTION 



31 



reached by the sperm before the shell is deposited. Some sharks and a 

 few teleosts are viviparous; copulation and "internal fertihzation" 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 cloacal glands of the male. The spermato- 

 phore 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. 



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

 reptiles and birds the egg-shell necessitates copulation and internal 

 impregnation. 



Modern mammals, except Ornithorhynchus and Echidna, are vivi- 

 parous. The two exceptional animals lay eggs of reptilian sort. There- 

 fore 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 immedi- 

 ately after fertilization. Therefore, if fertiliza- 

 tion 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 develop- 

 ment. 



Provisions for protection, nutrition and res- 

 piration during the period of development are 

 most diverse. 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. 30) and pipe-fish, the male carries the 



developing eggs in a brood-pouch on the ventral drawn after Boulenger in 

 surface of the body or tail— an arrangement Jt'to^yT^""^^' ^^'"'^^ 

 suggestive of the marsupial pouch of a female 



kangaroo. The smaller fish eggs, scantily endowed with yolk, develop 

 rapidly and soon become free-living and self-supporting while still very 

 minute. The miniature fish then enters upon a long period concerned 

 mainly with feeding and growth. Eggs containing larger quantities of 

 yolk pass through a longer period of development and the young fish 

 attains relatively large size before it is obliged to obtain food from an 



BROOD-POUCH- 



FiG. 30. — Sea-horse 

 (Hippocampus.); male, 

 with brood-pouch. (Re- 



