424 ARTHUR ROBINSON. 
of the epiblastic portion of the inner mass, and the cells of 
the outer layer over the surface of the inner mass assume a 
cubical or low columnar form, whilst the remainder of the 
wall of the blastocyst is formed by flattened cells. It is at 
this period that the hypoblast is differentiated from the inner 
part of the inner mass, and in these early stages (figs. 21 and 
22) the connection between it and the flat-celled portion of 
the blastocyst wall appears to be just as intimate as the con- 
nection of the latter with the cells of the outer layer which 
le over the inner mass and take part in the formation of the 
epiblast, and it is not possible to determine from the figures 
mentioned where the hypoblast ends. 
In the shrew, also, Hubrecht has shown (23) that the 
germinal epiblast and the hypoblast are formed from the inner 
mass of the blastocyst, which is very similar to the blastocyst 
of the rabbit and of the mole, but the fate of the outer layer 
over the inner mass is uncertain. Hubrecht says that the 
hypoblast-cells spread over the inner surface of the epiblastic 
wall of the blastocyst, and that they are distinguishable by their 
appearance, but his illustrations (pl. xxxv) are far from being 
convincing; they are, indeed, open to the same objections 
which have already been urged against the figures repre- 
senting the development of the rabbit. 
Selenka’s representations of the appearances observable in 
the early stages of development of the guinea-pig’s ovum lend 
themselves very readily to the support of the idea that the 
greater portion of the blastocyst wall is formed by the hypo- 
blast, for in Taf. xi, figs. 8, 5, 6, 7, and 9, in the fourth part 
of ‘Studien tiber Entwickelungsgeschichte der Thiere’ (45), 
the connection between the hypoblastic layer which is invagi- 
nated into the cavity of the blastocyst by the germinal epi- 
blast, and by the formation and the extension of the ‘‘ Inter- 
amnionhohle,” and the layer of flattened cells which forms the 
greater portion of the blastocyst wall is, at all events, quite 
as intimate as the connection of the latter layer with the 
epiblastic “ Trager.” 
In the opossum Selenka has demonstrated the appearance 
