THE EARLY DEVELOPMENT OF THE MAMMAL 419 



commonly somewhat larger than in the higher Mammals, are 

 sometimes of no greater size. The embryo has a brief intra- 

 uterine period of development, during which nutritive relations 

 are established with the uterine wall by means of the surface of 

 the chorion, which, however, retains its originally smooth sur- 

 face and merely comes closely into contact with the vascular 

 uterine epithelium, without acquiring a close organic union. 

 The yolk-sac is very large in these forms and underlies nearly 

 the entire chorion (serosa). Nutritive substances from the 

 maternal circulation may thus pass, with some difficulty, 

 through the chorion and the wall of the yolk-sac, into the blood 

 of the latter. 



Conditions suggestive of the higher Mammals are by no means 

 lacking, however, for in Dasyurus (Hill) the yolk-sac in certain 

 areas becomes very vascular and forms a close relation with 

 the uterine wall. The ectoderm cells of the chorion, between 

 the two, aid in establishing this relation between the maternal 

 and the embryonic blood, a relation which is very different from 

 the mere contact relation of the typical Metatheria. And in 

 Perameles (Hill) the allantois takes up a similar relation with 

 the uterine mucosa, sending into the latter w r ell-developed 

 vascular outgrowths (Fig. 174). We have here then, a con- 

 dition that in many respects resembles closely the relation 

 found in many of the "placental" Mammals. Indeed, these 

 structures are known as the "yolk-sac" and the "allantoic 

 placenta" respectively. 



Among the remaining Mammalian orders, or Eutheria (Mono- 

 delphia or Placentalia), there is the greatest diversity in the 

 mode and degree of the relation between embryonic membranes 

 and appendages, and the uterine wall, or, in other words, in 

 the character of the placentation. In the simpler cases (pig, 

 horse, and many others) the relation is much like that described 

 in Perameles in its essentials, while in the more highly specialized 

 instances (apes and man) the relation becomes very complex, 

 involving considerable modification of what is regarded as the 

 typical arrangement of the embryonic appendages. Between 

 these two extremes there is the greatest variety of conditions 



