88 MAMMALIAN DESCENT. [Lect. III. 



of the uterus. The 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 Carcliarias (white and blue sharks)." — 

 Balfour, o^;. 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 South America ; 

 afterwards call at the Southern States of the Xorth, 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 whmed." 



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-sacy 

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

 is seen in Reptiles and Birds, which have tlie 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 aUa7itois 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 {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, all these 

 membranes are developed, but there is scarcel}' any food-yolk, much 

 less, indeed, relatively than in Fishes and Amphibia that lay verj^ 

 small eggs. 



In the Eutheria the non- vascular amnion 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 yolJi-saCy 

 as large as, and even larger than, the other two meml)ranes, 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 temporary. 



