436 



GENERATION. 



egg is proportionally large, the foetus grows 

 principally by the assimilation of materials 

 procured from it, and there is not that intimate 

 connection of structure nor interchange of 

 substance between the mother and foetus which 

 occurs in Mammalia. The term ovo-viviparous 

 is applied to the variety of reproduction now 

 under consideration; as expressing that in it, 

 although the foetus is produced fully formed 

 and alive, the ovum is merely hatched within 

 the parent body. We find, accordingly, that 

 this form of generation is liable to vary, and 

 occasionally to run into the truly oviparous 

 kind. Thus, some animals which bear live 

 young at one season lay eggs at another season 

 of the year, as occurs in some Insects ; and 

 in others, as Lacerta agilis, the ova remain 

 within the mother's body for a part only of the 

 time employ ed in the development of the young ; 

 the process of hatching beginning and going 

 on for a longer or shorter time within the 

 female parent, and being completed as in the 

 bird without. 



In all truly viviparous, in most ovo-vivi- 

 parous, and also in many oviparous animals, 

 the ova are fecundated within the parent's 

 body ; and we find provided for the purpose 

 of introducing the seminal fluid into the female 

 genital organs, a more or less complicated ap- 

 paratus in both male and female, by which 

 the union of the sexes is brought about. In 

 the greater number of strictly oviparous ani- 

 mals, and particularly those that are aquatic, 

 as osseous fishes and Batrachian Reptiles, 

 fecundation is operated externally to the pa- 

 rent body ; that is, there is no union of the 

 sexual organs of the male and female, but the 

 ova laid by the female are covered with a 

 certain quantity of seminal fluid shed by the 

 male.* 



Utero-gestation in the Mammalia is termi- 

 nated by parturition or the birth of the young; 

 while in the bird or oviparous animal birth con- 

 sists in the exclusion of the young from the egg. 

 At this period in Mammalia, the organic con- 

 nection between mother and offspring is dis- 

 solved, and in both viviparous and oviparous 

 animals the birth is accompanied by important 

 structural changes, which fit the offspring for 

 independent life and aerial or aquatic respira- 

 tion. The young of Mammalia after birth, 

 though they cease to be organically connected 

 with the mother, continue to derive a certain 

 quantity of support from her, feeding on the 

 milk secreted by the mammae. But in all 

 other classes of animals, the young are at birth 

 capable of feeding on external aliment. 



Varieties in respect to utero-gestation and 

 the developement of the young. There is con- 

 siderable variety among different animals in 

 the degree of perfection at which the young 

 have arrived at the period of birth. Thus, 

 Insects, Batrachian Reptiles, and some other 



* In the land Salamander, which is ovo-viviparous 

 and breeds in the water, although there is no sexual 

 union of the male and female, there is yet internal 

 fecundation, the seminal fluid being carried into 

 the oviduct of the female along with the water in 

 which it is effused. 



animals leave the egg at a very early period ; 

 differing widely from their parents in struc- 

 ture and functions, they live for a time in a 

 masked or larva condition, and undergo after- 

 wards various changes or so-called metamor- 

 phoses before attaining the mature condition. 

 Fishes leave the egg while their structure is 

 yet very incomplete ; and even in the higher 

 animals we observe varieties in this respect: 

 thus, some birds, more especially those build- 

 ing on trees, are unfledged, blind, and help- 

 less when the shell is broken; and some 

 quadrupeds, among which may be mentioned 

 the Rodentia, Feline, and Canine species, 

 are at birth blind and weak, and with little 

 power of supporting their natural high tem- 

 perature.* The most remarkable instance 

 of variety of the kind now alluded to, how- 

 ever, occurs in the Kangaroo and other Mar- 

 supial animals, the generation of which de- 

 serves more particular mention in this place 

 as it exhibits a considerable deviation from the 

 more ordinary reproductive process in Mam- 

 malia, and is attended with some important 

 modifications in the structure of the generative 

 organs. 



In the Mammalia generally, it has already 

 been stated that there is an intimate organic 

 connection between the foetus and mother, by 

 means of which the former is supplied with 

 the materials of its growth. The intimacy of 

 this union (as we shall explain more fully else- 

 where) varies much in different tribes of ani- 

 mals. It is greatest in the placenta of the 

 human female, and from this there may be 

 traced a series of animals in which it becomes 

 more and more loose. According to recent 

 researches in Comparative Anatomy, there is 

 also observable in the descending series of 

 these animals a nearer and nearer approach in 

 the general structure of the body, and in the 

 conformation of the generative organs to the 

 oviparous type. This approach to the ovipa- 

 rous structure is most strongly marked in the 

 Marsupiata, as the Opossum, Kangaroo, &c. 

 and in the Monotremata, as the Ornithorynchus 

 and Echidna.f 



Mursupiute generation. The foetus of the 

 marsupiate animal leaves the uterine system of 

 the mother or is born at an early period of its 

 formation, while it is yet of a very small size, 

 and its organs are comparatively imperfectly 

 formed. On being born it is introduced by 

 the mother into the pouch or marsupium 

 formed by a reduplication of the integuments 

 of the lower part of the belly, and a short time 

 after it gets there, the foetus is found attached 

 by its mouth to one of the nipples of the 

 mamma!, which are concealed within the mar- 

 supium. The young of the marsupiate animal 



* Under the head of OVUM we shall shew that 

 all animals undergo changes which constitute me- 

 tamorphoses of one kind or other during their for- 

 mation or development. 



+ See the interesting papers by Mr. Owen on 

 the Generation of the Kangaroo and Ornithoryn- 

 chus in the Philosophical Transactions, part ii. 

 for 1834, p. 333, and in the Transactions of the 

 Zoological Society, vol. i. p. 221. 



