88 MAMMALIAN DESCENT. [Lect. III. 



of the uterus. Tlie yolk-sack becomes in this 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, op. cit., 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 we voyage across to Sou.th America ; 

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

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

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

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

 pig whined." 



A few words more about the Metatheria ; 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 embryonic membranes — yolk-sac, 

 amnion, and allantois — 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, although the fishy Frog 

 gets a rudiment of the allantois during transformation. But in 

 Eeptiles 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 (uterus) ; this 

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

 alive, as in the Vipers. In the Eutheria, or higher Mammals, aU these 

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

 less, indeed, relatively than in Fishes and Amphibia that lay very 

 small eggs. 



In the Eutheria the non-vascular anmion and the highly-vascular 

 allantois are highly developed, and it is the latter membrane, and not 

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

 perfect union and inter-communion with the equally vascular lining 

 of the enclosing organ. 



But the Marsupials have, as I have said, a moderately large yolJc-sac, 

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

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

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



