Till! PO !' MAMMALS. 223 



Three tacts can be cited in i'a \oiir of thi- hypoth-- 



In the tir.-t place, in the low- t' Mammals, as in the 



Monotivme- ami Mar-Mpials, t lit- e^- are larger than in placent:il 

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

 which, as in ( >rnit horhynchu.- for in-tance, i- depo.-it e 1 in cl 



compacted spheres of varying size and fat-like lu-tre. In tliis par- 

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

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



division of the Mammalia, are oviparous, like I'.irds anl lleptiles. 

 Quite recently two investigators, HAACKE and CALDWKU.. liave made 

 the interesting discovery that Echidna and Omit horhynchu-. in-i -,-id 

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

 w hich 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 r>ird.-, although the development takes place in the uterus. As 

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

 .minion. po.-esses a very large vascular yolk-sac, which ext mis out to 

 the aerosa, ami in addition a small allantois ami a sena. Tin- 1. 

 lies closely applied to the walls of the uterus, hut 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 

 Ultra-uterine nutrition begins to be cstahlished in th-> M:rsupials: 

 but otherwise the embryo with its envelope- lie- in the cavity of the 

 uterus, like the Avian or Reptilian embryo with its membrane- in 

 the firm egg-shell. 



Having established the hypothesis, already rxpiv.--ed by vaiiou- 

 authors, that the eggs of Mammals must originally have contained 

 more yolk, let us turn to a more exact description of the t'u-tal 

 membranes. As regards the tir>t stages of development, let us begin 

 with the I la hbit, because its embryology has been the most thoroughly 

 -iiLfatrd; then, in order to facilitate our understanding of the 

 structure of the human placenta, we shall show in a brief sketch how 7 , 

 in the cla of .Mammalia, in variou- uay> more intimate anatomical 

 and physiological relation- are developed between the mucous mem- 

 brane of the uterus ami the embryonic membrane-. \\V -hall treat 

 of the fu-tal membranes of Man in a >pecial chapter. 



