82 .T. P. HILL. 



formative region derived from tlie lower cell-ring of the same 

 stage. 



(d) Later History of the Two Regions of the Blasto- 

 cyst Wall (for details see pp. 72-74).— From the embryonal 

 region are derived the embryonal ectoderm and the entire 

 entoderm of the vesicle. I conclude, therefore, that it is tlie 

 homologue of the inner cell-mass or embryonal knot of the 

 Eutherian blastocyst. The extra-embryonal region directly 

 furnishes the outer estra-enibryonal layer of the vesicle wall, 

 i.e. the outer layer of the omphalopleure and chorion of later 

 stao-es. Assuming, as the facts of comparative anatomy and 

 palaeontology entirely justify us in doing, that the Mammals 

 are monophyleticand of reptilian origin, and further assuming 

 that the foetal membranes are homologous structures through- 

 out the Amniotan series (also in my view a perfectly 

 justifiable assumption)^, then the homologies of this extra- 

 embryonal region of the Marsupial blastocyt are not far to 

 seek. It is clearly the homologue of the extra-embryonal 

 ectoderm of the f^auropsidan and Monotreme egg, and the 

 homologue also of the outer enveloping layer of the Eutherian 

 blastocyst, to Avhich Hubrecht has given the special name of 

 'Hrophoblast." lu my view the trophoblast is none other 

 than extra-embryonal ectoderm which in the viviparous 

 mammals, in correlation with the intra-uterine mode of 

 development, has acquired a special significance for the 

 nutrition of the embryo. 



These, then, are my conclusions, and to me they seem on 

 general grounds perfectly obvious, viz. : (1) that the em- 

 bryonal or formative region of the unilaminar Marsupial 

 blastocyst is the homologue of the inner cell-mass or 



' How Asslietou ciin niaintaiu ("09. p. 2()<)) " that the amnion of the 

 rahljit is not more liomoloyous to the anniion of the Sauropsidan than 

 the lioniy teeth of Ornithorhynclnis are liomologous to the true teeth 

 of the mammal or reptile, which they liave supplanted," how he can 

 liold this view and yet proceed to utilise the presence of the amnion as 

 one of the leading characters distinguishing the Amuiota from the' 

 Anamnia, I fail to comprehend. Surely the presence of a series of 

 purely analogous structures in a group is of no classificatory value. 



