482 HENRY F. OSBORN. 



opossum it develops rapidly, so that a brief union with the 

 subzonal membrane before birth is not improbable. In the 

 unknown Marsupial (see Specimen 2) this union seems actually 

 to have taken place. 



3. The amnion, as in the Placentalia, in all cases invests the 

 embryo. 



4. One or two long furrows are formed along the lower 

 internal border of the uterus in the kangaroo and opossum. In 

 close contact with one of these in the opossum is placed the 

 villous chorionic disc of each of the numerous foetuses ; the 

 remaining portions of the subzonal membrane are free. The 

 embryo is undoubtedly retained in this position throughout 

 intra-uterine life. During this period the opossum is known 

 to keep remarkably quiet, so that the uterus is little disturbed, 

 and is most of the time in a horizontal position.^ The presence 

 of foetal villi is strong evidence by analogy of the presence of 

 minute crypts on the inner wall of the uterus. 



It is an undoubted inference from the above facts that in the 

 early stages of Marsupial development the vessels of the yolk- 

 sac not only are the channels for conveying the maternal nu- 

 triment to the foetus, but that this function is performed by 

 capillaries distributed in low villi, and separated from the 

 maternal structures, whatever the arrangement of the latter 

 may be, by an extremely thin layer of subzonal epithelium. It 

 is evident that these villi are altogether similar in structure to 

 those which are found over the allantoic chorion of the pig ;^ 

 the difference is merely one of degree. The rudimentary me- 

 chanism is sufficient to support the rapid growth of the embryo 

 opossum, which at birth is completely equipped with all the 

 necessary respiratory and digestive apparatus acquired during 

 an intra-uterine period barely exceeding two weeks.^ Tiiis 



1 The fact noticed by several observers, that the females are found in plenty 

 immediately after the birtii of the young, would seem to indicate that they 

 Lad been in hiding for some time. 



2 See Turner, loc. cit. 



^ The feebleness of the young at birth has been exaggerated. The opossum 

 young weigh from four to five grams, and in their bent position arc one half 



