106 J. p. HILL. 



which the embryo will be built up enclosed iu a cellulav 

 vesicle (the troplioblast), or which no part ever enters into 

 the einbr3'ouic organisation." The common possession by the 

 Metatheria and Eutheria of a larval membrane is after all 

 ouly what niio-ht be expected, "since after Hill's ('97) 

 investigations, we must assume that the didelphian mammals 

 are not descended from Ornithodelphia but from monodelphian 

 placental ancestors." As concerns the Prototheria, altliough 

 they cannot in ;iny sense be regarded as directly ancestral to 

 the other mammals, we nevertheless find the trophoblastic 

 vesicle "comparatively distinct." "In many reptiles and 

 birds," however, it is " distinguished with great difficulty 

 from the embryonic shield," and this is explained by the 

 fact that the Saui'opsida which are assumed to have taken 

 their origin from the same Protetrapodous stock as the 

 mammals but along an entirely independent line, have 

 secondarily acquired, like the Prototheria, the oviparous 

 habit, with its concomitants, a yolk-laden egg and a shell, and 

 this latter acquisition has naturally tended "to relegate any 

 outer larval layer to the pension list" ('09, p. 5). "Con- 

 cerning the yolk accumulation in the Sauropsidan egg, there 

 is no trouble at all to suppose that the vesicular blastocyst 

 of an early viviparous ancestor had gradually become yolk- 

 laden. The contrary assumption, found in the handbooks, 

 that the mammalian egg, while totally losing its yolk, has 

 yet preserved the identical developmental features as the 

 Sauropsid, is in reality much more difficult to reconcile with 

 sound evolutionary principles" ('09, p. 5). 



Amongst the lowoi- A^ertebrates the larval membrane is 

 clearly enough recognisable in the so-called Deckschicht of 

 tlie Teleostomes, Dipnoans, and Amphibians. It is frankly 

 admitted that Amphioxns, the Cyclostomes, and the Elasmo- 

 branchs " show in their early development no traces of a 

 Deckschicht" (larval layer, trophoblast), but there is no 

 difficulty about this, since it is easy enough to suppose, in 

 view of other characters, that " the Selachians may very well 

 have descended from ancestors without any outer larval layer " 



