STUDIES IN MAMMALIAN EMBRYOLOGY. 289 
deeper layer, which will become the epiblast of the germinal 
area, has been thus split off from the thickened polar knob of 
the blastocyst, the further growth of both the divisions be- 
comes independent. The lower stratum of embryonic epi- 
blast, connected with the wall of the blastocyst along a circular 
. line, increases in size together with the blastocyst, and appears 
in general very soon to become a layer that runs parallel to 
the curvature of the blastocyst. 
In the early stages it would appear that this epiblastic plate 
(in consequence of its mode of origin) is strongly bent inwards. 
In very many preparations I find it like this, though acknow- 
ledging that the amount to which it has bent may perhaps 
have somewhat increased during preservation. It very much 
resembles stages of the embryonic epiblast in Arvicola 
arvalis, as figured by Selenka (1. c. Pl. XVI, fig. 51.) 
The position of the cells composing it is the well-known 
arrangement that has been so often described in vertebrate 
development; in the early stages more than one layer of cells 
may certainly be said to be present, the component cells inter- 
wedging. As the embryonic disc increases in size this feature 
is gradually lost, and the epiblast is then a layer of only one- 
cell thickness. The circular line of its insertion on the peri- 
pheral epiblast of the blastocyst enlarges simultaneously with 
the total growth of the embryo. Whether at the margin the 
splitting process goes on for a short time, or whether the whole 
increase in size should be ascribed to simple growth, after the 
central splitting phenomenon has once occurred, cannot for the 
moment be definitely made out. At all events, I can find no 
evidence that any further marginal splitting occurs when once 
the mesoblast is being formed. The possibility of such a 
process will, however, be clear by a glance at figs. 15 a and 
17 a, Pl, XVI. 
This attachment of the disc of the embryonic epiblast against 
the epiblastic periphery of the blastocyst is lost when the 
amnion is formed. I must needs say a few words about this 
phenomenon in this place, although the regular demonstration 
would first require the discussion of the development of the 
