. Chordate anatomy. Chordata; Anatomy, Comparative. 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
. Chordate anatomy. Chordata; Anatomy, Comparative. 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 s
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