The Mammals and Man 



the higher mammals, and everything points to some marsupial 

 type as a connecting link between the egg-laying Monotremes 

 and the placental mammals. 



As has previously been indicated, the most important character- 

 istic of the third great group is a modification of the membranes 

 of the embryo to form a connection between it and the wall of the 

 uterus. The allantois develops as a highly vascular membrane, 

 the small blood vessels of which are brought into very close con- 



FIG. 113. Squirrel-like marsupial {Phascologale penicillata). 



tact with those in the wall of the uterus. So that, while the 

 blood of the mother does not actually mix with that of the child, 

 the two fluids are separated only by thin membranes, through 

 which nutritive substances easily pass. The broad advantage 

 of this is, of course, that the young animal passes the earlier 

 stages of its life inside the mother's body, where it is exposed to 

 a minimum of risk, is efficiently nourished, and from which it is 

 not sent forth into the world until it is tolerably well able to look 

 after itself. 



We shall now ask the reader to conceive a primitive placental 



133 



