88 MAMMALIAN DESCENT. [Lect. III. 



of the nteni.s. The yolk-sack becomes in tliis way firmly attached to 

 the walls of the uterus, and the two together form a kind of placenta. 

 A similar placenta is found in Carcharias (white and blue sharks)." — 

 Balfour, oj'). cif., p. 54. 



Now, having looked at the smooth and blue and white Sharks, we 

 seek the shore again, and are, once more, in Australia, among Kan- 

 garoos and Phalangers ; then Ave voyage across to South America ; 

 afterwards call at the Southern States of the North, and lastly, in 

 imagination, get home once more ; for the Hedgehog, the INIole, and 

 the Shrew will require our attention. Indeed, whilst writing about 

 the Marsupials, it seemed to me that " Thrice and once, the Hedge- 

 pig whine<l." 



A few Avords more about the INIetatheria ; after them the Edentata 

 must be spoken of ; and then we can look at the prickly infants of 

 the Hedge-hog. 



The Marsupials have all three; enibrjamic membranes — yollc-sacy 

 amnion, and aUantois — sub-equally developed, but not equal to what 

 is seen in Eeptiles and Birds, which have the yolk-sac as large as in 

 the Sharks and Skates but have, besides, the two other membranes, 

 which have no existence in fishes, as such, altliough the fishy Frog 

 gets a rudiment of the aUantois during transformation. But in 

 Reptiles and Birds the egg-coverings, membranous mostly, or both 

 membranous and calcareous, intervene between the developing embryo 

 and its inner enfoldings, and the walls of the egg-duct (ufenis) ; this 

 membrane may burst in the act of layijuj, and thus the young be born 

 alive, as in the A^ipers. In the Eutheria, or higher Mammals, all these 

 membranes are developed, but there is scarcely any food-yolk, mucli 

 less, indeed, relatively than in Eishes and Anqihilna that lay very 

 small e<fgs. 



In the Eutheria the non-vasculav anniion and the highly-vascular 

 allantois are highly develoi)ed, and it is the latter membrane, and not 

 the yolk-sac, as in the Shark, wliich forms that wonderful and most 

 perfect union and ii^iter-communion with the e(pially vascular lining 

 of the enclosing organ. 



But the Marsupials have, as I have said, a moderately large yoJk-saCy 

 as large as, and even larger than, the other two membranes, and this 

 forms, for a time, a commercial union with tlu." walls of the uterus in 

 a manner like that of the Shark, but this union is only temporary. 



