PHYSIOLOGY OF SEX AND REPRODUCTION. 229 



the fertilized egg has begun to develop, the mouth of the uterus is 

 closed by a secretion, which prevents the entrance of other spermato- 

 zoa should further copulation occur. (3) The period of gestation, that 

 is, between the fertilization of the ovum and the extrusion of the foetus, 

 varies widely in mammals, from about 18 days in opossum, or 30 in 

 rabbit, to about 280 days in Homo or 600 in the elephant, being- 

 longer in the more highly evolved types. But it also depends on size, 

 being about 280 days in cow and 150 in sheep; on number of off- 

 spring, being about 350 in mare and 60 in dog ; and on the degree of 

 maturity at birth, being 420 in giraffe and 40 in kangaroo. 



V. Parturition. — In many cases, for example, marine annelids, 

 mature ova burst, as we have already noted, from the mother animal, 

 who may thenceforth have nothing more to do with them. Liberation 

 of ova from the ovary and from the organism may be almost coinci- 

 dent, as in most bony fishes. In other cases, the ova are retained 

 within the mother until fertilized, 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 

 nutritive connection between mother and offspring. The term is of 

 little use, however, for the cases to wh ; ch it is applied shade off toward 

 the two other forms of birth. Thus among gristly fishes {Mustelus 

 Icevis and Carcharias), in the curious bony fish Anableps, and in certain 

 lizards {Trachydosauras and Cyclodus), a somewhat placenta-like 

 function is discharged by the yelk-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 recognized that the difference 

 between egg-laying and the production of young actively alive is only 

 one of degree. Even in mammals, which are viviparous par excellence, 

 the two lowest genera — the duck-mole and the echidna — are ovipar- 

 ous. The common grass-snake, normally oviparous, has been induced, 

 in artificial conditions, to bring forth its young alive, and this is prob- 

 ably true of other forms. The parthenogenetic generations of 



