THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 119 



enveloping layer which produces this peculiar entypic condi- 

 tion in the Eutherian blastocyst, I would interpret, then, as 

 a pui-ely adaptive phenomenon, wliich in the given circum- 

 stances effects in the simplest possible way the early completion 

 of the blastocyst wall, and whose origin is to be traced to 

 that reduction in size and in its envelopes which the Eutherian 

 ovum has suffered in the course of phylogeny, in adaptation 

 to the conditions of intra-uterine development. In particular, 

 starting with a shell-bearing ovum, already minute and 

 undergoing its development in utero, I see in the loss of 

 the shell such as has occurred in the Eutheria an intelligible 

 explanation of the first origin of those adaptations which 

 culminate in the condition of entyp3^ I aui therefore wholly 

 unable to accept the vicAV of Hubrecht ('08, p. 78), that ''what 

 Selenka has designated by the name of Entypie is — from 

 our point of view — no secondary phenomenon, but one 

 which repeats very primitive features of separation between 

 embryonic ectoderm and larval envelope in invertebrate 

 ancestors." 



I see no reason for supposing that the intimate relationship 

 which is early established in many Eutheria between the 

 trophoblastic ectoderm and the uterine mucosa has had any- 

 thing to do with the origination of the entypic condition. In 

 my view such intimate relationship involving the complete 

 enclosure of the blastocyst in the mucosa only came to be 

 established secondarily, after entypy had become the rule. 

 On the other hand, the peculiar modifications of the entypic 

 condition met with in rodents with "inversion" (e.g. rat, 

 mouse, guinea-pig) are undoubtedly to be correlated, as Van 

 Beneden also believed ('99, p. 332), with the remarkably early 

 and complete enclosure or implantation of the germ in the 

 mucosa such as occurs in these and other Eutheria. Similar 

 views are expressed by Selenka in one of his last contributions 

 to mammalian embryology. He writes ('00, p. 205) — " Dass 

 die Entypie des Keimfeldes und die Blattinversiou begiinstigt 

 wird durch die friihzeitige Verwachsung der Eiblase mit dem 

 Uterus, ist nicht in Abrede zu stellen. Aber da dieser 



