. THE FCETAL MEMBRANES OF MAMMALS. 223 



Three facts can be cited in favour of this hypothesis. 



In the first place, in the lowest classes of Mammals, as in the 

 Monotremes and Marsupials, the eggs are larger than in placental 

 animals. They are characterised by a large quantity of yolk, 

 which, as in Ornithorhynchus for instance, is deposited in closely 

 compacted spheres of varying size and fat-like lustre. In this par- 

 ticular they form a transition to the eggs of Reptiles and Birds. 



Secondly, it has been observed that the Monotremes, the lowest 

 division of the Mammalia, are oviparous, like Birds and Reptiles. 

 Quite recently two investigators, HAACKE and CALDWELL, have made 

 the interesting discovery that Echidna and Ornithorhynchus, instead 

 of giving birth to living young, as was hitherto assumed, lay eggs 

 which are nearly two centimetres in diameter, and enveloped in a 

 parchment-like shell, and which they carry about with them in their 

 brood-pouch or mammary pocket. 



Thirdly, the foetal membranes of Marsupials, which next to the 

 Monotremes are to be considered as the lowest Mammals, remain 

 permanently in a condition which corresponds to that of Reptiles 

 and Birds, although the development takes place in the uterus. As 

 we know through OWEN, the embryo, which is enclosed in a capacious 

 amnion, possesses a very large vascular yolk-sac, which extends out to 

 the serosa, and in addition a small allantois and a serosa. The latter 

 lies closely applied to the walls of the uterus, but without being 

 intimately united with it. Probably, therefore, after resorption of 

 the yolk, substances which have been secreted by the uterus are 

 taken up by the blood-capillaries of the yolk-sac. Thus a kind of 

 intra-uterine nutrition begins to be established in the Marsupials ; 

 but otherwise the embryo with its envelopes lies in the cavity of the 

 uterus, like the Avian or Reptilian embryo with its membranes in 

 the firm egg-shell. 



Having established the hypothesis, already expressed by various 

 authors, that the eggs of Mammals must originally have contained 

 more yolk, let us turn to a more exact description of the foetal 

 membranes. As regards the first stages of development, let us begin 

 with the Rabbit, because its embryology has been the most thoroughly 

 investigated; then, in order to facilitate our understanding of the 

 structure of the human placenta, we shall show in a brief sketch how, 

 in the class of Mammalia, in various ways more intimate anatomical 

 and physiological relations are developed between the mucous mem- 

 brane of the uterus and the embryonic membranes. We shall treat 

 of the foetal membranes of Man in a special chapter. 



