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Part III. — Eighth Annual Report 
The outermost cells form a well-marked layer, the epiblast, though 
whether this designation of the layer as alone forming the epiblast 
is perfectly justifiable, is open to question. It appears certain that 
many of the lower cells have arisen from the outer layer. Below the 
outer layer lies a mass of cells which shows no order ; the cells resting 
on a concave depression of the yolk. In the yolk itself {y.c.) many cells 
formed in it, and in process of wandering into the blastoderm are seen. 
Besides these elements lying in the yolk, a number of 'merocytes' — 
many more than in the preceding stage, are also met with. These cells 
to which Eiickert assigns a role in the formation of blood-corpuscles, have 
been more fully investigated by Ziegler, who has pointed out that they 
never show the phenomenon of ' indirect ' or karyokinetic cell division, 
but that they undergo 'fragmentation.' He holds that they are, in fact, 
degenerating cells or nuclei, and that they are concerned in causing or 
producing some changes in the yolk itself. Ziegler has shown that the 
blood is formed from quite another source. 
It is interesting to recall in this connection the 1 fibrine ferment ' of the 
blood, which, according to A. Schmidt and Gamgee, is produced from the 
nuclei of the white corpuscles, and which induces the coagulation of the 
blood. In a similar fashion the ' merocytes ' may be supposed to produce 
some ' ferment ' by their degeneration, which initiates such changes in 
the yolk as render it fit for absorption, and of use to the embryo in 
building up its tissues. 
The succeeding stage (stage iv. of Kastschenko's nomenclature) is 
figured in surface view of the entire germinal disc in fig. 4^ 
It is more particularly described as the stage of a visible segmentation 
cavity (s.e., figs. 3 and 4). In this figure (in section in fig. 3 along the 
line marked in fig. 4) a great aggregation of the lower cells has taken 
place towards one end of the germinal disc. This is the preliminary to 
the formation of the embryo, and in the following stage we shall see the 
first traces of the future embryo formed at this thickened end. 
The segmentation cavity (s.c.) is bounded on the outside partially by 
'epiblast/ and partially by segmentation cells, inside and laterally by 
segmentation cells (fx.) and the yolk (y). 
In fig. 5 we have a surface view of the first rudiments of the embryo, in 
the form of a semi-lunar thickening of the rim of the germinal disc (e.t.). 
This embryo, which may reach this stage in two or three weeks during the 
summer months, requires five or six weeks for its development to the 
same point in the cold winter months. 
Development now goes on somewhat more rapidly, and we soon see an 
outgrowth or knob from the central portion of the semilunar embryonic 
thickening towards the centre of the germinal area (fig. 6). The axis of 
this is the long axis of the future skate, and its pointed end is the anterior 
or head end. 
In fig. 7,* which shows only the embryonic portion of the blastodermic 
area, the process has grown apace, and now forms a lancet-shaped curved 
plate. The embryo, for so it may be called, grows forwards at the expense 
of the lateral thickenings of the embryonic rim (l.r.). The groove (b) at 
the hinder end of the disc is part of the blastopore, still open over the 
yolk. 
The next figure (fig. 8) is characterised by the commencing formation 
of the medullary groove (m.g.) along the axis of the embryo. It forms a 
spoon-shaped depression, which does not yet reach to the anterior end of 
the embryo. The figure corresponds to Balfour's stage C. 
In fig. 9 (IX of Balfour) the anterior end of the embryo has broadened 
* Figs. 5 and 7 correspond to stages A and B of Balfour's nomenclature. 
