ON THE ZOOLOGICAL STATION AT NAPLES. 457 
followed in the same individual so easily and satisfactorily. The slowness 
of development ensures us not missing anything we want, without the in- 
convenience of prolonged or all-night sittings. I thus followed one animal 
for over two months, when he succumbed, probably to the hot weather. 
This period began a week before the embryo made its appearance on 
the blastoderm, and at the end all the gills were represented (stages A to 
K of Balfour). 
This plan is not merely interesting, it is usefalin two ways. It enables 
one to know with some precision when to open an egg for any particular 
stage ; and secondly, we thus learn some things that we cannot any other 
way. Balfour, forinstance, more than once refers to the segmentation cavity 
as first appearing towards the non-embryonic end of the blastoderm, and 
to a preliminary thickening occurring towards the embryonic end.! Now 
this is certainly the view one would be led to adopt, without some system- 
atic observation of the same blastoderm by transmitted light at daily 
intervals. There is a swelling towards the opposite end of the blastoderm 
to that at which the segmentation cavity arises, but this does not come to 
anything, and the segmentation cavity, so to speak, travels as it increases 
in size from the end at which it first appeared towards the other. At 
least although the behaviour of the segmentation cavity varies, sometimes 
spreading from one end over the whole blastoderm, the above is what 
often occurs, and this change of position is the explanation of Balfour’s 
statement, the main point being that, whatever variation there may be, 
the embryo always appears at the end of the blastoderm at which the first 
trace of the segmentation cavity had previously been observed. 
I would again use a reference to Balfour to give weight to my next 
remark as to the method of preserving blastoderms prior to or just about 
the time of the first appearance of the embryo. Balfour says,” ‘The 
shape of the blastoderm in hardened specimens is not to be relied upon, 
owing to the traction which the blastoderm undergoes during the process 
of removing the yolk from the egg shell.’ Now the method recommended 
is this: Do not remove the yolk from the egg shell, but carefully holding 
the egg with its broadest section horizontal, and having made sure that 
the blastoderm is turned to the uppermost side of the yolk, cut away the 
overlying side of the shell, and then, using the fingers, not forceps, place 
the egg carefully in Kleinenberg’s picro-sulphuric acid and follow Bal- 
four’s instructions for the chick. Leave the egg for four or five hours in 
Kleinenberg, then with all care replace this by alcohol of 30 per cent. to 
wash, and this again by 50 per cent. for one hour, and only after the egg 
has been in this for the whole or part of the time attempt to cut round 
the blastoderm. We are thus enabled to remove the blastoderm 
preserved flat and undistorted and in its natural relations to the 
underlying yolk, a greater or less thickness of which can be removed 
with it, according as the alcohol has acted a longer or shorter 
time. It must have been the non-adoption of some such method as this, 
coupled with the fact that the microtomes of that time did not, I believe, 
give facilities for making complete series of sections which are so desir- 
able, that prevented Balfour from ascertaining the very interesting fact 
(a fact for which he was obviously on the look-out), first hinted at by 
Riickert * in Torpedo, and described for Pristiwrus a few months ago by 
1 Hlasm. Fishes, pp. 26, 33, 34, 44, 46, 54. 2 Op: ott. MD. T1. 
’*Ueber die Anlage des mittleren Keimblattes und die erste Blutbildung bei 
Torpedo,’ Anat. Anz. 1887. 
