RELATIONS OF YOLK TO GASTRULA IN TELEOSTEANS. 11 
This stage is illustrated in figs. 4 and 14, the latter represent- 
ing simply an optical section, while the former gives also a 
projection of the part of the blastoderm between the eye and 
the plane focussed. Fig. 15 shows a surface view of the blas- 
toderm at the stage in question. The form of the blastoderm 
is now no longer symmetrical; in the neighbourhood of one 
radius from its centre it is considerably thicker than elsewhere, 
and this thickened part is the first indication of the embryo. 
It will be seen, when the history of the blastoderm is traced 
on to its later stages, that the part of the embryo which is thus 
early formed is the dorsal region of the head. 
Soon after the first appearance of the segmentation cavity a 
distinct layer of cells is seen extending inwards from the edge 
of the blastoderm towards the centre. This layer is in contact 
with the periblast below and with the blastoderm above ; it is 
continuous externally with the upper layer of the blastoderm, 
and thus appears to be formed by a growth inwards of the edge 
of the blastoderm. The relations of this layer, as seen in living 
ova, are shown in figs. 5 and 16. The invaginated layer, as it 
is usually called, does not extend over the floor of the segmen- 
tation cavity, but is confined to the peripheral region of the 
blastoderm. For this region the name embryonic ring may be 
conveniently used. The segmentation cavity maintains the 
same relations as at the previous stage, except that it is some- 
what flattened, and more eccentric in position: the whole 
blastoderm has extended and covers a larger area of the yolk 
than before. The embryonic thickening has become longer 
and more distinct. The invaginated layer extends further 
inwards in the neighbourhood of the embryonic rudiment than 
beneath the rest of the edge of the blastoderm. In fig. 17 is 
represented the appearance of the surface of the blastoderm 
after the formation of the invaginated layer: the lighter 
region marks the position and extent of the segmentation 
cavity, and the darker, the area occupied by the invaginated 
layer. The embryonic rudiment forms a blunt projection 
inwards towards the segmentation cavity, while the outer con- 
tour of the blastoderm is regularly circular. 
