716 



ANATOMY OF VERTEBRATES. 



566 



Left uterus impregnated, Ornithorliynchus. lxxvii'. 



imbedded in the soft, thick, plicated, smooth-surfaced, and well- 

 organised lining membrane of the uterus. In a second Ornitho- 

 rhynchus, shot in the same locality, on the 7th of October, the 

 ova, fig. 566, c, c, from the two discharged ovisacs, ib. b, b, were 



situated a little below 

 the middle of the left 

 uterus ; they were also 

 spherical, each three 

 lines in diameter, of a 

 lighter colour than the 

 preceding, specially at 

 the upper part, from 

 the subsidence of the 

 contained vitelline or 

 germinal mass : they 

 were smooth, and rolled 

 freely out of the posi- 

 tion where they were 

 lodged. In a third 

 specimen, shot on the 

 evening on which the first specimen was obtained, the uterine ovum 

 had the same spherical form, smooth exterior surface, and freedom 

 from connexion with the uterus ; but was of a 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 was situated in a depression or cell a little below 

 the middle of the left uterus. The lining membrane of the 

 uterus was much thickened and highly vascular in each of the 

 above specimens. In all these ova the contents were of two 

 kinds, viz. a greyish sub-transparent fluid, and a yellowish denser 

 mass, which varied in their relative proportions as above-men- 

 tioned : in the largest ovum, the yellow mass, germ or yolk, 

 occupied about one-third of its cavity, while in the smallest it 

 constituted four-fifths of the whole mass. The membrane, which 

 may be the hyalinion or ( zona pellucida' of the ovarian ovum, but 

 which I would still, as in 1834, call ' chorion,' 1 offers a moderate 

 degree of resistance when torn open, and yields equally in every 

 direction when separated from the yolk, the rent margins 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. The fluid, 



1 This term signifies the ' outer tunic ' of the uterine ovum : it may be ' zona ' or 

 something laid upon the zona, or something superseding the zona, sneh as the animal 

 layer of the blastoderma, or the outer or vascular layer of the allantois. 



