GENERATION. 



435 



ences in the structure of the other parts of the 

 body. 



In Hermaphrodite animals there are two 

 modes in \vhich fecundation takes ^place. In 

 some of the Acephala, and in the Holothuriae, 

 the union of the sexual organs necessary for 

 fecundation takes place in a single individual; 

 while in others, as Helix and Lymneus among 

 the Gasteropoda, copulation, or the union of 

 two individuals, is required, and there is mutual 

 impregnation, the female organ of each animal 

 being fecundated by the male of the other, 

 a mode of impregnation which also exists in 

 the common Earth-worm, Leech, and some 

 other animals. Occasionally we find that three 

 or more individuals engage in this sort of mu- 

 tual fecundation, being arranged in a chain or 

 circle.* (See HERM APHRODITE.) 



Dioecious reproduction, or ivit/i distinct in- 

 dividuals of different sexes. Oviparous and 

 viviparous generation. In those animals again 

 in which the position of the sexual organs on 

 separate individuals renders copulation neces- 

 sary, the mode of production of the new animal 

 from the egg seems to be the most prominent 

 circumstance according to which the reproduc- 

 tive process is modified. Thus, while in a 

 certain number of them the young are born 

 alive, in others they are hatched from eggs laid 

 by the female parent. This constitutes the 

 difference between Viviparous and Oviparous 

 animals ; to the first of which classes Mam- 

 malia belong, to the second Birds, and most 

 Reptiles and Fishes. A short comparison of 

 the more important steps of the generative 

 process in the Mammiferous animal and the 

 Bird will most readily explain the difference 

 between viviparous and oviparous generation.! 



* In the Cyclostoma viviparum the sexes are 

 distinct. 



t Harvey in the Sixty-third Exercitation thus 

 enforces the analogy between the oviparous and 

 viviparous modes of reproduction. " I have 

 already given you the reason why I have drawn 

 out documents concerning all other egges from 

 the egges of Hens ; namely, because they are 

 cheap and every man's purchase. " " But there 

 is more difficulty in the search into the gene- 

 ration of viviparous animals ; for we are almost 

 quite debarred of dissecting the humane uterus : 

 and to make any inquiry concerning this matter in 

 Horses, Oxen, Goats, and other Cattel, cannot be 

 without a great deal of pains and expense. But 

 those who are desirous to make tryal whether we 

 deliver the truth or not, may essay the business in 

 Doggs, Conies, Cats, and the like." " But we in 

 the entrance of these our observations have con- 

 cluded that all animals are in some sort produced 

 out of an egg : For the fo2tus of viviparous creatures 

 is produced after the same manner and order out of 

 a pre-existent conception, as the chicken is formed 

 and constituted out of an egge. There being one 

 and the same species of generation in them all, 

 and the exordium or first principle of them all is 

 either called an egge, or at lest something answer- 

 able and proportionable to it. For an Egge is an 

 exposed conception from which a Chicken is pro- 

 duced ; but a conception is an egge retained within, 

 untill the fcetus have attained its just bulk and 

 magnitude : in other matters it squares with an 

 egge," &c. " Besides, as a Chicken is hatched out 

 of an Egg, by the fostering heat of the sitting Hen, 

 or some other ascititious hospitable patronage, so 

 also the foetus is produced out of the conception in 



In both these classes of animals ova are 

 formed from the ovary, and in both the ova 

 are fecundated within the body of the female 

 parent. The process by which the egg is sepa- 

 rated from the place of its formation, and the 

 changes it undergoes in being perfected after 

 this separation, are the same in both : but after 

 the fecundation and completion of the egg, it is 

 differently placed in the two classes of animals ; 

 for in birds the egg passes through the oviduct 

 and leaves the body of the female parent, to be 

 hatched into life under the influence of favour- 

 able external agents ; while in the mammiferous 

 quadruped, the egg remains within the uterus 

 of the female generative organs, becomes at- 

 tached to it, and has there formed from it the 

 young animal, which does not quit the body of 

 the parent until it is capable of independent 

 life. The egg of the bird leaves the body of 

 the mother provided with a considerable quan- 

 tity of organic matter, by which alone, under 

 the influence of heat and air, the embryo is 

 nourished during incubation. The egg of the 

 mammiferous animal is extremely small com- 

 pared to the size of the young animal at birth, 

 and the fcetus consequently draws a continual 

 supply of the materials of its nourishment from 

 the uterus of the mother, with which it is more 

 or less intimately connected. The residence of 

 the child or young animal in the body of the 

 mother during its formation and growth is 

 termed pregnancy, or utero-gestation.* 



Ovo-viviparous generation. There are other 

 animals, however, besides Mammalia, which 

 bear their young alive, as is the case in many 

 cartilaginous and a few osseous fishes, in 

 several Batrachia, Sauria, and Ophidia, and 

 also in some Gasteropodous Mollusca, Insects, 

 Annelida, and Entozoa. But there is an im- 

 portant difference to be pointed out between 

 the viviparous form of generation occurring in 

 these animals and that which belongs to the 

 Mammalia. For the female generative organs 

 of the above-mentioned animals, as well as 

 the eggs they produce, resemble much more 

 closely in their structure those of oviparous 

 than those of strictly viviparous animals. As, 

 in the animals now under consideration, the 



the egge, by the soft and most natural warmth of 

 the parent. " And then, concerning that which 

 relates to procreation, the foetus is produced out of 

 the conception in the selfe same manner and order 

 as the chicken out of the Egg ; with this only dif- 

 ference, that in an egge, whatever relates to the 

 constitution and nutrition of the Chicken, is at once 

 contained in it ; but the conception ( after the foetus is 

 now formed out of it) doth attract more nourishment 

 out of his parent's womb j whereupon the nourish- 

 ment increases with the foetus." " The first Con- 

 ception, or Rudiment, therefore, of all Animals is 

 in the uterus," (this applies to Quadrupeds,) 

 " which, according to Aristotle, is like an egg co- 

 vered over with a membrane when the shell is 

 pilled off." And Harvey finally concludes with 

 Aristotle : "All animals, whether they be swimming, 

 walking, or flying animals ; and whether they be 

 born in the form of an Animal or of an Egge ; are 

 all generated after the same manner." 



* The nature of the egg of viviparous animals, 

 which has only recently been fully understood, 

 will be described in a subsequent part of this paper, 

 and more in detail in the article OVUM. 



