248 THE EVOLUTION OF SEX. 



cases, the ova are retained within the mother until fertiUsed, 

 but are expelled not long after, before development has advanced 

 to any marked degree. Such eggs are often furnished with the 

 important capital of nutriment, so familiar in the case of birds, 

 and may be also surrounded by chitinous, horny, membranous, 

 or limy shells. All such forms of birth are familiarly described 

 as oviparous. 



In numerous invertebrates, fishes, amphibians, and reptiles, 

 the ova develop within the mother, and the young are born 

 more or less actively alive. To such cases, where there is no 

 nutritive connection between parent and offspring, the term 

 ovo-viviparous used to be applied. They were contrasted with 

 oviparous birth, as in birds, on the one hand, and with the 

 viviparous birth of mammals, on the other. It is the well- 

 known characteristic of the latter that there is an intimate nutri- 

 tive connection between mother and offspring. The term is of 

 little use, however, for the cases to which it is applied shade off 

 towards the two other forms of birth. Thus among gristly 

 fishes {Mustelus Icevis and Carcharias), in the curious bony 

 fish Anabkps, and in certain lizards {Trachydosauriis and 

 Cydodus), a somewhat placenta-like function is discharged by 

 the yolk-sac and the wall of the oviduct ; while in fishes, 

 reptiles, &c., oviparous and ovo-viviparous birth may occur in 

 nearly related forms. The distinction involved in the term is 

 therefore abandoned, and it must also be recognised that the 

 difference between egg-laying and the production of young 

 actively alive is only one of degree. Even in mammals, which 

 are V\Y\\):ixous par excellence^ the two lowest genera — the duck- 

 mole and the echidna — are oviparous. The common grass- 

 snake, normally oviparous, has been induced, in artificial condi- 

 tions, to bring forth its young alive, and this is probably true of 

 other forms. The parthenogenetic generations of aphides are 

 usually viviparous, while the fertilised eggs are laid as such. 



§ 5. Early Nutrition. — The early nutrition of the embryo, 

 and even larva, is in most cases an absorption of the legacy of 

 yolk material, which is probably richest in the eggs of birds. 

 The tadpole of the frog grows and exerts itself for some time 

 before it begins to feed at the expense of this inheritance of 

 yolk. Later on, in the frog division of amphibians, the growth 

 of new structures appears to be provided for by the nutritive 

 absorption of the tail, the larva literally living upon itself. The 

 same is true in the elaborate metamorphosis of echinoderm 



