12, ep) and an under layer; that the
upper is a coherent membrane of colum-
nar nucleated cells, but that the lower
one (Fig. 12, 7) is formed of an irregular
network of larger cells in which the
nuclei are with difficulty visible; and
that in addition to this there are certain
still larger cells, called ‘formative cells’
(Fig. 12, b), lying at the bottom of the
segmentation-cavity.
Under the influence of incubation
‘ changes take place very rapidly, which


II1.] THE HYPOBLAST. 51


result in the formation of the three layers of the blasto-
derm.
The upper layer, which is the epiblast already
spoken of (Fig. 13), takes at first but little share in
these changes.
In the lower layer, however, certain of the cells
begin to get flattened horizontally, their granules become
less numerous, and the nucleus becomes distinct; the
cells so altered cohere together and form a membrane.
The membrane thus formed, which is first completed in


Fie, 13.
Vr y
TIS ee Eee


TRANSVERSE SECTION THROUGH THE BLASTODERM OF A CHICK
BEFORE THE APPEARANCE OF THE PRIMITIVE STREAK.
The epiblast is represented somewhat diagrammatically. The
hyphens shew the points of junction of the two halves of the
section. The hypoblast is already constituted as a membrane of
flattened cells, and a number of scattered cells are seen between
it and the epiblast.
the centre of the pellucid area, constitutes the hypoblast.
Between the hypoblastic membrane and the epiblast
there remain a number of scattered cells (Fig. 13) which
cannot however be said to form a definite layer altogether
distinct from the hypoblast. They are almost entirely
confined to the posterior part of the area pellucida, and


4—2,


52 THE FIRST DAY. [CHAP.
