MONOTREMATA. 



397 



with a smooth and polished surface, and had 

 not the slightest adherence to the uterine pa- 

 rietes. 



The specimen containing the two ova next 

 in size (fg. 192, c c) was shot in the same 

 locality on the 7th of October. These ova 

 measured each three lines in diameter, and 

 were situated a little below the middle of the 

 left uterus: they were of a. spherical form, 

 but had evidently been slightly compressed in 

 the uterine cavity. They were of a lighter 

 colour than the preceding; a circumstance 

 which was specially evident at the upper part, 

 from the subsidence of the contained vitellme 

 mass. Externally they were smooth and rolled 

 freely out of the position where they were 

 lodged, like those of the preceding specimen. 



The third specimen, in the uterus of which 

 the largest ovum was contained, was shot on 

 the evening on which the first specimen was 

 obtained. This ovum had the same spherical 

 form, smooth exterior surface, and freedom 

 from connexion with the uterus, as in the pre- 

 ceding; but was of a much lighter colour, 

 owing to the increased quantity of its fluid 

 contents, to which its greater size was chiefly 

 attributable. It measured three lines and a 

 half in diameter, and had been situated in a 

 depression or cell a little below the middle of 

 the left uterus. The lining membrane of the 

 uterus was highly vascular in the recent state 

 in each of the above specimens. 



In all these ova the contents could be seen, 

 through the cortical or outer membrane, to be 

 of two kinds, viz. a greyish sub-transparent 

 fluid, and a yellowish denser mass, which 

 varied in their relative proportions as above- 

 mentioned, the denser substance always sub- 

 siding to the lowest part of the ovum, which- 

 ever way it was turned. 



In the largest ovum, the yellow mass or 

 yolk occupied about one-third of its cavity, 

 while in the smallest it constituted four-fifths 

 of the whole mass. 



The chorion or cortical membrane of these 

 ova (f'g- 193, ) offered a moderate degree 

 of resistance when torn open with the forceps, 

 and yielded equally in every direction when 

 separated from the yolk, the rent margins 



Fig. 193. 



Fig. 194. 



Uterine Ovum, magnified and dissected, 



Ornithorhynchus. 

 (Owen, Phil. Trans. 1834. ) 



curling inwards like the coat of an hydatid. 

 This membrane is of a dull greyish colour, 

 inclining to brown, slightly transparent, and 

 more polished upon its inner than upon its 

 outer surface : it resembles the cortical mem- 

 brane of the ovum of the Salamander, but is 

 of a more delicate texture. The fluid contents 

 occupied the space between the cortical and 

 vitelline membranes, a situation analogous to 

 that of the albumen in the egg of the fowl, 

 but had not become coagulated by the action 

 of the spirit in which it had been so long im- 

 mersed. 



The yellow matter, or yolk, was seen to be 

 invested by its proper capsule (fig. 193, 6), 

 which, when reflected under the microscope, 

 was found to consist of an extremely thin, 

 smooth, and transparent outer layer, which 

 I regard as the membrana vitelli (Jig. 194, ), 

 with a thicker granular membrane immediately 

 lining it, analogous to the blastoderma or ger- 

 minative stratum (fig. 194, b). 



The contents of the 

 above investments, or 

 substance of the yolk, 

 consisted of innume- 

 rable minute opaque 

 granules, similar in size 

 and regularity of form 

 to those contained in 

 the ovarian follicles ; 

 and with these gra- 

 nules were mingled 

 larger transparent glo- _ 



bules of oil. There was P f on ^** vitelline mem- 

 .1 i-i. orane and germinal stra- 



not the slightest trace tum> Ornithorhynchu,. 

 of chalazae attached to (Owen,Phil.Trans.l384.) 

 the vitelline membrane, 



as from analogy we might have expected, had 

 the ovum been destined to have been perfected 

 by incubation. I was unable to detect any 

 rudiments of the embryo : an opaque streak 

 was discernible on one part of the yolk, but 

 not sufficiently definite to be satisfactorily re- 

 cognised as a cicatricula; it is indeed, proba- 

 ble, from the observation of Lieutenant Maule,* 

 that the ova attain a greater size by the im- 

 bibition of the nutrient material before the 

 lineaments of the foetus become visible. 



The changes which the impregnated uteri of 

 the Ornithorhynchus had undergone, as com- 

 pared with the same part in the quiescent state, 

 were greater than those which have been ob- 

 served to take place in the Kangaroo. The 

 uterus containing the two smallest sized ova 

 measured seven lines in diameter, but was 

 much firmer and denser than in the unim- 

 pregnated specimens ; and having also in- 

 creased in length, was thrown into more abrupt 

 curves on either side of the ovarian ligament. 

 The uterus which had contained the largest 

 ovum measured an inch in diameter; and that 

 containing those of the second size was of 



* Proceedings of the Zoological Society, 1832. 

 ' In the insides of several female platypi which 

 were shot, eggs were found of the size of a large 

 musket-ball and downwards." 



