THE DEVELOPMENT OF ORGANISMS 795 



oviparous mammals — the duckmole (Ornithorhynchus) and the 

 Spiny Anteater (Echidna). The duckmole lays two eggs in a nest 

 in a burrow; the Spiny Anteater, after liberating its single egg, 

 takes it in its mouth and places it in a temporarily developed skin 

 pouch. The eggs of these "Monotremes", as the oviparous mammals 

 are called, contain a considerable quantity of 3^olk, which is 

 practically absent from the much smaller ova of other mammals. 

 The ova of ordinary mammals are usually about the size of small 

 pinheads, but those of the oviparous types of Monotremes, to 

 which we have referred, are |-J inch when they pass from the 

 ovary into the oviduct, and i by | inch when they are extruded. 

 This relatively large size, which means that there is a considerable 

 quantity of yolk, is obviously correlated with the fact that the 

 development of those Monotreme eggs takes place for the most part 

 outside of the mother. When the young one is hatched, whether in 

 the nest (Duckmole), or in the pouch (Echidna), it begins to lick 

 the mother's skin on the area (on the under surface or in the pouch) 

 bearing the numerous apertures of the milk glands, which are devoid 

 of teats or mammae. In these and in many other ways the Mono- 

 tremes are obviously very old-fashioned. Perhaps we should notice 

 that most authorities distinguish a third genus, Proechidna, also 

 oviparous, and nearly allied to Echidna. This detail does not affect 

 our argument. 



Among Marsupials there seems to be only one type, the tree 

 bandicoot or Perameles, which has the true (allantoic) placenta that 

 is found in aU ordinary mammals. The other marsupials do not get 

 beyond a makeshift (yolk-sac) placenta, which also occurs as a 

 transitory stage in the gestation of Rodents, Insectivores, and Bats. 

 Thus the ante-natal partnership between the Marsupial mother 

 and her offspring is not so intimate as that obtaining in ordinary 

 mammals. Not only so, the gestation is very short— in some cases 

 only a fortnight. In the large species of Kangaroo [Macropus 

 giganteus), which may stand as high as a man, the period of gesta- 

 tion is only thirty-nine days; and then the young one is born, 

 blind and naked, very imperfectly finished, and not much more than 

 an inch in length! The prematurely born young marsupials are 

 peculiarly helpless, but they have the instinct to climb about on 

 the under side of the mother and to enter the skin-pocket or marsu- 

 pium if they succeed in reaching it. Indeed, they are so helpless 

 that they cannot even suck the mammae to which they become 

 attached inside the pouch, which is developed in most, though not 

 all, of the female Marsupials. The milk has to be forced down their 

 gullet by the contraction of a special (cremaster) muscle around the 

 mammary glands; and there is an interesting adaptation-^the 

 shunting forward of the top of the windpipe into the posterior 

 nares — which enables the young one in the pouch to continue 



