THE EARLY DEVELOPMENT OF THE MAESUPIALL\. 105 



of Ampliioxus, the Cyclostomes, and tlie Elasmobranchs) are 

 descended from "verinifortn predecessors of coelenterate 

 pedigree" possessed of free-swimming larvas, in which there 

 was present a complete lai-val membrane of ectodermal deriva- 

 tion, find of the same order of differentiation '' iis tlie outer 

 larval layer which in certain Nemertines^Gephyreans, and other 

 worms often serves as a temporary envelope that is stripped 

 off when the animal attains to a ceitain stage of development." 

 When, for oviparity and larval development, viviparity and 

 embryonic development became established in the Prote- 

 trapodous successors of the ancestral vermiform stock, the 

 larval membrane did not disappear. On the contrary, it is 

 assumed that it njerely changed " its protective or locomotor 

 function into an adhesive one,'' and so, development now 

 taking place in utero, it is quite easy to understand how the 

 larval membrane could gradually become transformed into 

 a trophic vesicle, containing the embryo as before, and 

 functional in the reception of nutriment from the walls of 

 the maternal uterus. The final stages in the evolution of 

 this trophic vesicle constituted by the old larval membrane 

 are met with amongst the mammals, since in them it 

 became vascularised so as to constitute a "yet more 

 thorough system of nourishment at the expense of the 

 maternal circulatory system." Such, then, is the phylogeny 

 of the trophoblast according to Hubrecht. The Eutherian 

 mammaJs, which it is held trace tiieir descent straight back to 

 some very early Protetrapodous stock, viviparous in habit and 

 with small yolk-poor, holoblastic eggs, exhibit the tropho- 

 blast in its most perfect condition. Hubrecht therefore starts 

 with them, and attempts to demonstrate the existence of a 

 larval membrane, or remnants of such, externally to the 

 embryonal ectoderm in all vertebrates with the exceptions 

 already mentioned. There is no question of its existence in 

 the Meta- and Eutherian mammals. " We may," writes 

 Hubrecht ('08, p. 12), . . . "insist upon the fact that 

 . . . all Didelphia and Monodelphia hitherto investi- 

 gated show at a very early moment the didermic stage out of 



