70 J. p. HILL. 



gap which has hitherto existed in our knowledge of its early 

 ontogenesis. Although the mode of origin of the entoderm 

 in Dasyuvns would appear, in the present state of our know- 

 ledge, to find its closest parallel, not amongst vertebrates, but 

 in certain invertebrates (cf . the mode of origin of the ento- 

 dermal cells from the wall of the blastula in Hydra as 

 described by Brauer^), the observations of Assheton ('94) 

 on the early history of the entoderm in the rabbit, when 

 viewed in the light of the foregoing, seem to me to afford 

 ground for the belief that phenomena comparable Avith those 

 here recorded for Dasyurus Avill eventually be recognised as 

 occurring also in Eutheria. 



Hubrecht ('08), in his recent treatise on early Mammalian 

 ontogeny, deals very briefly with the question of the origin 

 of tlie entoderm in the latter group, merely stating that 

 " from the inner cell-mass arises by delamination a separate 

 lower layer which we designate as the entoderm of the 

 embryo. These entoderm cells wander in radial direction 

 along the inner surface of the trophoblast, which in many 

 cases is thus soon transformed into a didermic structure, 

 i . . When the entoderm has separated off by delamina- 

 tion from the embryonic knob, the remaining cells of the 

 latter form the * embryonic ectoderm,' which is thus situated 

 between the entoderm and the trophoblast." 



Assheton, in the paper just referred to, has given a careful 

 account of the first appearance of the entodermal cells in the 

 7-abbit, and of what he believes to be the mode of their 

 peripheral extension below the trophoblastic wall of the 

 blastocyst. He shows that the inner cell-mass, at first 

 spherical, gradually, as the blastocyst enlarges, flattens out 

 below the " covering layer" of the trophoblast until it. forms 

 an approximatel}^ circular plate " nowhere more than two 

 cells thick." During the process of flattening, cells are seen 

 to jut out from the periphery of the mass; these eventually 

 separate,, and appear as rounded cells scattered irregularly 

 over the inner surface of the trophoblast and ''extending 

 ' ' Zeitscbr. f. wiss. Zool.,' Bd. lii, 1891. 



