

DEVELOPMENT OF MONOTREMATA. 



717 



Uterine Ovum, magnified and dissected, 

 Ornithorhynchus. lxxvii'. 



568 



answering to that which appears between the yolk and zona pellu- 

 cida after impregnation in the Rabbit's ovum (fig. 562, marked by 

 the arrows), occupies 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 immersed : it divides 

 the chorion, fig. 567, a, from the vitelline membrane, ib. b : this 

 membrane, fig. 568, a, is thin, smooth, 

 and transparent ; adherent to parts of 

 its inner surface was a thicker granular 

 layer, answering to the ( blastoderm,' 

 or germinal stratum, fig. 568, b. In 

 each of the above impregnated Mono- 

 tremes l the discharged ovisacs, fig. 

 566, by b, were of an elongate 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 substance escaped. 

 When longitudinally divided, they were found 

 to consist of the same parts as the ovisac 

 before impregnation ; but the theca, or inner- 

 most 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. 



On the 8th of December Dr. Bennett dis- 

 covered in the subterranean nest of an Orni- 

 thorhynchus three living young, naked, not quite two inches in 

 length, fig. 600. On the 12th of August (1864) a female 

 JSchidna hystrix was captured in the hollow of a prostrate 

 * cotton tree,' in Colac Forest, Victoria Province, Australia, 

 having a young one, fig. 603, e, with its head buried in a mam- 

 mary or marsupial fossa, ib. c. This young one was naked, of 

 a bright red colour, and one inch two lines in length. Between 

 the condition of the uterine ovum, as in fig. 567, and that of the 

 (probably new-born, or recently born) young Monotremes, above- 

 mentioned, I have not hitherto received materials for further 

 elucidating the development of the foetus in this singular group of 

 Mammals : whether cleavage of the yolk takes place prior to the 



1 lxxvii'. I was indebted to my old friend and fellow-student, George Bennett, 

 now F.R.S, for the above mentioned specimens. 



Portion of the vitelline mem- 

 brane and germinal stratum, 

 Oniithorliynchus. lxxvii'. 



