170 
F. M. BALFOUR. 
situated in relation to the embryo? Before attempting to 
answer these questions I will shortly describe the develop- 
ment, so far as I have made it out, for the stages during 
which the cumulus is visible. 
The first change that I find in the embryo (when examined 
after it has been hardened)^ is the appearance of a small, 
whitish spot, which is at first very indistinct. A section 
through such an ovum (PI. XX, fig. 10) shows that the cells 
of about one half of the ovum have become more columnar 
than those of the other half, and that there is a point {pr. c.) 
near one end of the thickened half where the cells are more 
columnar, and about two layers or so deep. It appears tome 
probable that this point is the whitish spot visible in the hard- 
ened ovum. In a somewhat later stage (PI. XIX, fig. 1) the 
whitish spot becomes more conspicuous (j»c), and appears as 
a distinct prominence, which is, without doubt, the primitive 
cumulus, and from it there proceeds on one side a whitish 
streak. The prominence,as noticed by Claparede andBalbiani, 
is situated on the flatter side of the ovum. Sections at this 
stage show the same features as the previous stage, except 
that (1) the cells throughout are smaller, (2) those of the 
thickened hemisphere of the ovum more columnar, and (3) 
cumulus is formed of several rows of cells, though not 
divided into distinct layers. In the next stage the appear- 
ances from the surface are rather more obscure, and in some 
of my best specimens a coagulum, derived from the fluid 
surrounding the ovum, covers the most important part of the 
blastoderm. In PI. XIX, fig. 2, I have attempted to repre- 
sent, as truly as I could, the appearances presented by the 
ovum. There is a well-marked whitish side of the ovum, 
near one end of which is a prominence {pc), which must, 
no doubt, be identified with the cumulus of the earlier stages. 
Towards the opposite end, or perhaps rather nearer the 
centre of the white side of the ovum, is an imperfectly 
marked triangular white area. There can be no doubt that 
the line connecting the cumulus with the triangular area is 
the future long axis of the embryo, and the white area is, 
without doubt, the procephalic lobe of Balbiani. 
A section of the ovum at this stage is represented in 
PI. XX, fig. 11. It is not quite certain in what direction 
the section is taken, but I think it probable it is somewhat 
oblique to the long axis. Ho\vever this may be, the section 
^ I was unfortunately too much engaged, at the time when the eggs 
were collected, to study them in the fresh condition ; a fact which has 
added not a little to my difficulties in elucidating the obscure points in 
the early stages. 
