422 Royal Society. 



The tail is much shorter than in the young Kangaroo, and shows 

 as much proportional size as in the full-grown Echidna, in which it 

 is a mere stump concealed hy the quills and hair. 



The head is proportionally longer and more slender in the mar- 

 supial foetus of the Echidna than in that of the Ornithorhynchus or 

 of the Kangaroo, and already at this early period foreshows the 

 characteristic elongation and attenuation of that part in the mature 

 animal. The form of the mouth, as a transverse slit, is a good mo- 

 notrematous character of the young at that period, since, in all true 

 or teated marsupials, the mouth of the mammary foetus has a peculiar 

 circular and tubular shape. A scarcely visible linear cicatrix at the 

 middle of the lower part of the abdomen is the sole trace of umbilicus. 



A bifid obtuse rudiment of penis or clitoris projects from the fore 

 part of the single urogenital or cloacal aperture, and in advance of 

 the base of the tail-stump. 



The brain, of which the largest part was the mesencephalon, 

 chiefly consisting of a vesicular condition of the optic lobes, had col- 

 lapsed at this part, leaving a well-defined elliptical fossa of the in- 

 tegument, indicative of the widely open fontanelle at the upper part 

 of the cranium. 



The skin of the shrunken body showed folds, indicative of the ori- 

 ginally plump, well-filled abdomen. 



The fore limbs, in their shortness and breadth, foreshow the charac- 

 teristics of those of the parent, which may be said, indeed, to retain 

 in this respect the embryonic character, with superinduced breadth 

 and strength. The digits have already something of the adult pro- 

 portions, the first or innermost of the five being the shortest ; the 

 others of nearly equal length, but. graduating shorter from the third 

 to the fifth. The characteristic disposition of the digits was better 

 marked in the hind limb, the second already being the strongest 

 and longest, the rest more rapidly shortening to the fifth than in 

 the fore leg. The innermost, agreeably with the law of closer re- 

 tention of type in the embryo, though the shortest of the five, was 

 less disproportionately so than in the adult. 



The chief points, in the generative economy of the Monotremes, 

 which still remain to be determined by actual observation are : — 



1. The manner of copulation. 



2. The season of copulation. 



3. The period of gestation. 



4. The nature and succession of the temporary structures for the 

 nourishment and respiration of the foetus prior to birth or exclusion. 



5. The size, condition, and powers of the young at the time of 

 birth or exclusion. 



6. The period during which the young requires the lacteal nou- 

 rishment. 



7. The age at which the animal attains its full size. 



In respect to the second point : as the female Echidna with the 

 young was captured on the 12th of iVugust, she might be impreg- 

 nated at the latter end of June or in July. Females, therefore, killed 

 in the last week of July and the first week in August, in the pro- 



