DEVELOPMENT OP THE HORSE. 7 



or inner tunic known as the amnion. The fluid lying between 

 the chick and the amnion (in the dark space in fig. 1) is most 

 useful in preventing jars when the egg is moved, and the thin 

 but tough skin forming the wall of the amnion prevents the 

 chick injuring the yolk sac and allantoic The figure also 

 indicates the position of the air space invariably found, and a 

 portion of the albumen (the white of the egg) which is gradually 

 absorbed as additional nourishment, partly by the yolk sac and 

 partly by the allantoic 



The Opossum's Foetal Appendages. 



The young mammal, like the chick, is invested by a fairly tight- 

 fitting tunic, the amnion (am., fig. 2); and, like the chick, it is 

 connected by one stalk to the yolk sac (y.s., fig. 2), and by a second 

 to an allantois (all., fig. 2). But, unlike the chick, the yolk sac in 

 the mammal never contains yolk, 1 and the allantois never plays 

 the part of a simple breathing organ — never, as in the bird, 

 collects oxygen direct from the air. If the yolk sac contains no 

 yolk, and the allantois is unable to expose the foetal blood to the 

 air, how does the embryo mammal live and breathe ? In the 

 opossum, and, in fact, in all mammals except the duckmole and 

 echidna, nourishment, at least to start with, is absorbed directly 

 from the uterus into the yolkless yolk sac. 2 This nourishment, 

 which is partly secreted by uterine glands, is in a sense com- 

 parable to the albumen in the bird's egg, and is probably an excel- 

 lent substitute for the crude material stored up in the yolk sac 

 of birds and lizards. But the embryo mammal not only requires 

 nourishment, it requires to be anchored for a longer or shorter 

 period to the inner wall or lining of the uterus. This fixing is 

 secured in, e.g., the opossum, by means of the thin outer shell-less 

 coat or tunic, which incloses the embryo and all its appendages. 



Fig. 2 represents in a diagrammatic way a young American 

 opossum. The embryo, invested by a thin inner tunic, the 



1 The duckmole and echidna are exceptions to this rule, but I agree with 

 Professor Hubrecht of Utrecht in believing that the monotremes are off the main 

 mammalian line. 



2 The uterus, in which development takes place, being able to provide nourish- 

 ment, it has been found unnecessary to store up food in a large yolk sac, as in 

 the bird, which for the sake of lightness is almost of necessity oviparous. 



