THE EARLY DEVELOPMENT OF THE MAKSUPL\LTA. 101 



trophoblast was originally employed by Hubrecht as a con- 

 venient term designatory of what he at the time regarded as 

 the extra-embryonal ectoderm of the mammalian blastocyst. 

 In the course of his speculations on the origin of this layer, 

 however, he has reached the conclusion that it is really of the 

 nature of " a larval envelope, an Embryonalhiille " {'08, p. 15), 

 inherited by the mammals, not from the reptiles (which have 

 no direct phylogenetic relationship to the latter), but from 

 their remote invertebrate ancestors ("vermiform predecessors 

 of ccelenterate pedigree, provided with an ectodermal larval 

 investment [Larvenhtille] "). 



Assheton, again, although he was unable to convince him- 

 self ('94) of the correctness of van Beneden's account of the 

 occurrence of a process of epiboly in the segmenting eggs of 

 the rabbit, finds in the sheep ('98) that a differentiation into 

 two groups of cells is recognisable " perhaps as early as the 

 eight segment stage," and that one of the groups gradually 

 envelops the other. "Let it be noted," he writes ('98, p. 227), 

 " that we have now to face the fact, based on actual sections, 

 that there is in certain mammals a clear separation of 

 segments at an early stage into two groups, one of which 

 eventually completely surrounds the other," and instances 

 Van Beneden's observations on the rabbit (of the correctness 

 of which he, however, failed to satisfy himself, as noted above), 

 Duval's observations on the bat, Hubrecht's on Tupaia, and 

 his own on the sheep. Assheton thinks this phenomenon 

 " must surely have some most profound significance," 

 but finds himself unable to accept the interpretations of 

 either Van Beneden or Hubrecht, and puts forward yet 

 another view, " based on the appearance of some segmenting 

 eggs of the sheep" ('OS, p. 233), "that in cases where this 

 differentiation does clearly occur, it is a division into epiblast 

 and hypoblast, the latter being the external layer" ('98, p. 227). 

 Assheton thus differs from all other observers in holding that 

 the inner cell-mass or embryonal knot of the Eutherian 

 blastocyst gives origin solely to the formative or embryonal 

 ectoderm, and I believe 1 am correct in stating that he also 



