J, W. Jenkinson 
149 
sagittal plane coincides with both. But the first furrow may be at right angles to 
the plane of symmetry, and in that case the sagittal plane may coincide with 
either or be in between. 
A complete treatment of the whole question demands therefore the determina- 
tion of tlie position in each of a large number of eggs of the sperm-path, the plane 
of symmetry, the first furrow and the sagittal plaTie. This is, however, un- 
fortunately impossible, since the sperm path disappears long before the appearance 
of the sagittal plane. It is possible, however, to determine either the first three 
or the last three in one and the same egg ; and this I have attempted to do. In 
the first part of this paper I can only give the results of the measurement of the 
angles between the plane of symmetry, the first furrow and the sagittal plane in a 
fairly large number of cases. The relation between the sperm path and the first 
furrow involves the cutting of a large number of eggs into series of sections, and 
must be left till later. 
The angle between the first furrow and the sagittal plane may be measured in 
various ways. The eggs may be fastened by the jelly to glass plates coated with 
paraffin and the direction of the planes marked, with the aid of a lens, by a needle 
on the wax. Or the eggs may be fastened direct to the underside of a glass plate 
and the directions marked with a paint-brush. These methods are, however, very 
inaccurate, and I have discarded all the measurements made in this and previous 
years in this way. They are inaccurate for two reasons. In the first place it is 
very difficult to place the marks accurately, and in the second, the sagittal plane 
has to be determined by the direction of the medullary folds or plate, a direction 
which is probably in many cases not the original direction of the median plane, 
for during the closure of the blastopore the eggs rotate not only about a horizontal 
axis but about a vertical axis as well, though of course to a less, and to an irregular 
extent. ... 
It became necessary therefore to determine the sagittal plane by the position 
of the dorsal lip of the blastopore in an early stage before rotation has begun. To 
do this the egg must be viewed from the lower side. I made use of the following 
apparatus. The microscope was placed with the tube horizontal, and to the stage 
was attached a holder which carried a slide on which the eggs were placed, and 
below this a mirror at 45". The eggs were illuminated from below. The slide 
was ruled with a diamond with parallel lines, and by means of the micrometer and 
goniometer with which the ocular was provided it was a simple matter to read 
off the angle between the first furrow and a line on the slide, and later on 
to determine the position of the sagittal plane in the same fashion. 
The same method was used for the plane of symmetry. 
One possible objection to this means of measurement is, I have satisfied 
myself, groundless. Between the first and the second measurements the eggs 
must be kept in a damp chamber, and it might be thought that in moving them 
