5 A. A. W. HUBRECHT. 
2. The Origin of the Entoderm. 
Soon a most important process is inaugurated and 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. 
Sometimes, as for instance in Tarsius (Hubrecht, ’02) the 
more important part of the delaminating entoderm (viz. that 
which remains situated below the rest of the embryonic-knob- 
cells) is present as a distinct cell-layer before this migra- 
tion of entoderm cells towards the inner surface of 
the trophoblast commences (Figs. 8, 19); in other cases 
(Sorex, Lepus, a. 0.) the entoderm cells migrate as soon as 
formed ; whereas in Tupaja it is only after the entodermic 
vesicle has become nearly completed that the part of it which 
will remain in contact with the embryonic ectoderm is sepa- 
rated off from the latter by delamination (Fig. 29). 
In certain mammals (T'arsius, monkeys, man) the ento- 
derm cells never clothe the whole of the inner surface of the 
trophoblast, the entodermic vesicle remaining of smaller size 
than the trophoblastic sphere (Figs. 39, 40, 44—46, 62—65). 
To a certain extent this is explained by the fact that another 
vesicle (of which mention will be made later on) develops, 
at an uncommonly early period, fills up part of the vesicle 
formed by the trophoblast and prevents the entoderm cells 
from reaching the entire outer surface.} 
When the entoderm has separated off by delamination from 
' It would seem that in Erinaceus a similar state of thing occurs tem po- 
rarily, it having been observed by me (’89, Figs. 25, 26) that a closed 
entodermic vesicle, far smaller than the trophoblastic sphere which encloses it, 
is here noticed in very early stages (Fig. 34). Shortly after this (Figs. 
35—38) the hedge-hog’s blastocyst, is, however, a spherical trophoblast, 
against which the endoderm is everywhere adherent. The investigation of 
numerous early stages of the hedge-hog is, however, yet necessary to settle 
this point. 
