ORGANS OF GENERATION IN THE VERTEBRATA. 103 



some peculiarity in their mode of generation. In all the other 

 mammalia the ovum is fecundated in the ovary, from which it de- 

 scends through the Fallopian tube to the uterus, in the higher 

 orders about the twelfth day. An intimate vascular connection is 

 now established between the ovum and the uterus, by means of the 

 placenta, and continues till the embryo is fully formed, and as we 

 say, capable of enjoying an independent existence. This consti- 

 tutes the period of utero-gestation, which, in the human subject is 

 ten lunar months, but varies in almost every species of the class. 

 A very different process, however, takes place in the animals above 

 alluded to, thus in the kangaroo, the foetus, small and imperfect, 

 leaves the uterus about the thirty-ninth day after conception, and 

 is lodged by the mother in the marsupium or pouch formed for its 

 reception on the lower part of the abdomen, by a fold of integument 

 in which the mammary gland is placed. Shortly after it reaches 

 the pouch, it is found attached by its mouth to one of the nipples, 

 from which it receives a constant supply of milk ; this is rendered 

 more secure by the gland being covered over by a stratum of mus- 

 cular fibres, which enables the mother to feed the foetus at pleasure. 



The mode of breeding of the monotremaia is as yet involved in 

 some obscurity. The word monotremata means animals with a 

 single outlet from the genito-urinary and digestive organs, called a 

 cloaca; and is applicable to all the marsupiata, but is at present 

 confined to the edentate species of them, as the echidna and orni- 

 thorhynchi. Previous to the late valuable researches of Mr. Owen, 

 the existence of mammary glands in these animals was denied, 

 which, combined with the structure of their ovaries, and their pe- 

 culiar development of uterus, led to the supposition of their being 

 oviparous in their generation. 



Although the structure of their generative organs, the presence 

 of mammary glands, their bearing their young alive, and suckling 

 them, confer on them the ovo-viviparous type of generation, it must 

 be acknowledged they possess many characters in common with 

 reptiles. 



RECAPITULATION. 



1. All forms of generation conform to the fissiparous, gemmipa- 

 rous, oviparous, viviparous, and ovo-viviparous. 



2. These organs are separate and distinct in the insecta, the 

 annelida, and the Crustacea. In the mollusca they are single, and 

 confined to one side. 



3. This system is simple in fishes, and a rudimentary penis oc- 

 casionally present. 



4. Internal impregnation rarely occurs among the amphibia or 

 reptiles. 



5. In birds the size of the testicles varies according to the season, 

 and the penis is often absent. 



