396 



MONOTREMATA. 



thorhynchus, is of a spherical form, 

 and very nearly fills the ovisac. The 

 diameter of the germinal vesicle is to 

 that of the ovum as 1 to 38. The 

 vitelline fluid is rich in the number 

 of its nucleated cells or granules, 

 and the intermixed, clear, colourless 

 oil-globules. The vitelline membrane 

 is moderately thick, smooth, and 

 highly refracting. The ovum is se- 

 parated from the ovarian vesicle, or 

 lining membrane of the ovisac, by a 

 very small quantity of fluid and a 

 stratum of granules or cells. The 

 proper tunic of the ovisac consists of 

 a dense and very vascular layer of 

 the ' stroma' or proper tissue of the 

 ovary, which is rather thicker and 

 more distinctly laminated than in 

 most Mammalia, and in this re- 

 spect widely differs from the lax 

 stroma of the ovary of the bird. The 

 most important difference to be noted in the 

 present comparison with the bird is the small 

 size of the ovarian ovum, depending on the 

 relatively scanty amount of vitelline matter, 

 superadded in the ovary to the essential part of 

 the ovum, the vesicula germmatha. 



It may be objected that the impregnated 

 ovarian ova in birds rapidly augment in bulk 

 as the time of dehiscence approaches, and that, 

 although the ovaria of the Ornithorhynchus 

 may have been investigated within a few days 

 of the reception of the impregnated ovum into 

 the oviduct, the changes occurring in such a 

 period might much more nearly approximate 

 the ovarian ovum of the' Ornithorhynchus to 

 the size and other conditions of that of the bird 

 than in the instances above described. 



The following observations on the impreg- 

 nated ovum in the uterus itself prove, how- 

 ever, that no such approximation to the bird 

 in regard to the proportion of yolk added to 

 the ovarian ovum, or as respects the size of 

 the ovum prior to dehiscence, is made by the 

 Ornithorhynchus. 



For the acquisition of this important evi- 

 dence in the question of the generation of the 

 Monotremata science is indebted to the ex- 

 ertions of Mr.Geo. Bennett, F.L.S., a Member 

 of the Royal College of Surgeons in London, 

 and Colonial Zoologist at Sydney, New South 

 Wales. Three uteri, containing undeveloped 

 ova, were transmitted by that gentleman to the 

 Museum of the Royal College of Surgeons, 

 in 1834, and with the sanction of the Board of 

 Curators, were described by me in the Philo- 

 sophical Transactions of that year.* 



In these specimens the left ovary only had 

 taken on the sexual actions, but did not ex- 

 ceed in size the same parts in the unimpreg- 

 nated specimens above described. The right 

 ovary had, however, become enlarged ; it mea- 

 sured half an inch in length, a third of an 

 inch in breadth, and was about half a line in 



* See vol. cxxiv. p. 555, and Physiol. Catalogue 

 of the Hunterian Museum, vol. v. p. 112, No. 

 3460 A. 



Fig. 192. 



Left uterus impregnated, Ornithorhynchus. 

 (Oiven, Phil. Tram. 1834. ) 



thickness : a few ovisacs, about the size of a 

 small pin's head, projected from the surface. 

 The left ovary in each of the specimens was 

 concealed by the thin membrane forming the 

 expanded orifice of the oviduct, to which it 

 was agglutinated by a coagulated secretion. 

 In two of the specimens the left ovary pre- 

 sented two empty ovisacs, or corpora lutea 

 (fig- 192, b 6), corresponding with the num- 

 ber of ova found in the uterus. In the third 

 specimen the left ovary presented two ovisacs 

 still uncicatrized ; but only one ovum was con- 

 tained in the uterus. In a fourth specimen 

 three similar ovisacs were present, but the ova 

 had been removed from the uterine cavity. 

 The discharged ovisacs were of an elongated 

 flask-shaped form about three lines in length, 

 and two in diameter, with the margins of the 

 orifice, through which the ovum and granular 

 substance had passed, everted, with a slight 

 contraction, resembling the neck of a flask, 

 below the aperture. On compressing these 

 ovisacs, small portions of coagulated sub- 

 stance escaped. When longitudinally divided, 

 they were found to consist of the same parts 

 as the ovisac before impregnation, with the 

 exception of the granular contents and gra- 

 nular stratum ; but the theca, or innermost 

 parietes of the sac, was much thickened, and 

 encroached irregularly upon the empty space, 

 so as to leave only a cylindrical passage to the 

 external opening. 



The impregnated Ornithorhynchus, in the 

 uterus of which the two smallest sized ova 

 were found, was shot on the evening of the 

 6th of October, 1832, in the Yas river, Murray 

 County, New South Wales. These ova were 

 of a semitransparent white colour when recent, 

 but had lost that appearance when examined 

 at the Museum, to which they had been trans- 

 mitted, in situ, with the uterus and surround- 

 ing parts well preserved in spirits. The ova 

 were situated at the upper part of the left 

 uterus, and at the distance of about a line 

 from each other. Each ovum was spherical in 

 form, and measured two lines and a half in 

 diameter; they were of a deep yellow colour, 



