182 A. A. W. HUBRECHT. 



never met witli in the fossil condition ! But they may, never- 

 theless, be all the more impartial judges when we have to 

 choose between two different assumptions: the one given in the 

 hand-books, according to which mammals must, through the 

 Ornithodelphia, be derived from some oviparous sauropsidian 

 ancestor, or the one here advocated, according to which a 

 viviparous Prototrapod, provided with an adhesive and dis- 

 tending larval layer diverged into various directions, some of 

 the descendants utilising the conditions of growth and develop- 

 ment (such as they find them) with the highest degree of 

 intensity and becoming primates, others applying their tropho- 

 blast to nutritive purposes in more diverse and less direct 

 ways, becoming the ancestors of most of our other Mono- 

 delphia and Didelphia. Others, again, going a certain distance 

 with the preceding, but then acquiring yolk-laden eggs 

 (Ornithodelphia), whilst yet other very effective branchings 

 off in various directions gave rise to the primitive sauropsidian 

 ancestors. 



The difference between the saui'opsidian and the amphibian 

 descendants of the protetrapods need no longer be so incisive 

 — as those zoologists that divide the Vertebrates into Amniota 

 and Anamnia would make it. The hypothesis here brought 

 forward proposes to look upon what we know as the Decks- 

 chicht of the early larval Amphibia and Dipnoi, and even of 

 the teleostomes, as a last remnant of the very larval layer from 

 which we started in trying to explain the foetal membranes of 

 vertebrates according to what seems to me a simple plan. 



We have now to look a little closer into certain details, by 

 which we may be enabled to judge of the greater or smaller 

 degree of tenability of some of the views here brought forward. 



We notice that all the Mammalia-monodelphia, that have 

 up to now been observed in very early stages, fully confirm 

 the strong antithesis which in those early stages prevails 

 between the trophoblast and the embryonic cells strictiori 

 sensu. We also notice this in the Didelphia, as far at least 

 as Selenka's figures for the opossum go, although he himself 

 has not interpreted the facts he brought to light in the same 



