THE MAMMALIA 307 



cells of the trophoblastic tissue of the placenta are found to 

 contain glycogen. The maternal glycogen was converted into 

 sugar, passed into the maternal blood circulating in the placental 

 lacunae, was there taken up by the trophoblastic cells, recon- 

 verted into glycogen, and once more converted into sugar and 

 passed into the embryonic circulation by way of the allantoic 

 blood-vessels. This complicated series of changes from glycogen 

 to sugar, and back again, emphasises the fact that, although 

 there is free exchange of material between, there is no actual 

 intermingling of, maternal and embryonic tissue except in the 

 case of the blood. Consequently when the foetus is born 

 i.e. expelled by the contractions of the uterus through the 

 vagina to the exterior the placenta, consisting of trophoblastic 

 and allantoic (i.e. embryonic) tissue, is pulled off from the 

 uterine wall and carried away with the foetus, forming the 

 so-called decidua. But no part.of the maternal tissue, excepting 

 the blood circulating in the trophoblastic lacunae, is carried 

 away at birth, previous statements to the contrary notwith- 

 standing. 



There is a good deal of variety in the mode of placentation 

 in the different orders of mammalia. The foregoing account 

 is chiefly applicable to the mouse, and it may be taken as 

 fairly typical of the orders Rodentia, Carnivora, Insectivora, 

 Cheiroptera, Monkeys, and Man. In the remainder of the 

 Eutheria the placenta is simpler. 



It will be noticed that in describing the placentation, con- 

 tinual reference has been made to a yolk-sac, and yet no food- 

 yolk is present in the ovum, and the sac in question is only a 

 vesicle with fluid contents. The fact is that the "yolk-sac" 

 of the mammalian embryo corresponds to the similarly named 

 organ in the embryos of reptiles and birds, which is filled with 

 food-yolk, and gradually absorbed during the growth of the 

 embryo. There is no doubt that many of the peculiar features 

 of the early stages of the development of mammals are to be 

 explained by their Derivation from an ancestor which laid 

 large-yolked eggs, and it should not be forgotten that the 

 lowest mammals, Echidna and Ornithorhyncus, do, in fact, lay 

 such eggs. 



