J. W. Jenkinson 
169 
TABLE XXIII. 
Sagittal Plane. 
Aunle 
A. Light tlirouKli- 
out from -90° 
B a. In darkness 
till segmentation, 
then exposed to 
light from -90° 
B /3. In darkness 
till dorsal lip 
-180—150 
7 
17 
27 
15 
\\ 
120—90 
15 
28 
28 
90—60 
19 
S7 
22 
60— SO 
22 
34 
31 
30-0 
27 
41 
47 
+ 0—30 
24 
43 
40 
30—60 
10 
34 
28 
60—90 
18 
29 
90—120 
14 
19 
22 
120—150 
17 
25 
25 
150—180 
15 
36 
25 
Totals 
199 
355 
335 
The very great interest attaching, I venture to think, to these results lies in 
the fact that it is experimentally possible to separate those factors in the constitu- 
tion of the fertilized ovum upon which the direction of cell-division depends from 
those which determine first the Plane of Symmetry and, next, the position of the 
median plane of the embryo, or the direction, if I may so express it, of differentiation. 
The first depends clearly on the pressure, the second on gravitation (and light). 
When both pressure and gravitation act upon the egg, and, as in the conditions of 
the above experiment, at right angles to one another, the deviations of the Plane 
of Symmetry and of the Sagittal Plane from the First Furrow will be large, the 
result actually obtained ; when, on the other hand, both agencies are eliminated, 
the agreement of these two planes with the First Furrow is closer. Light is also a 
disturbing factor, and vvei-e light, as well as those small interfering agencies — such 
as deformation of the eggs by natural pressure in the uterus, or by handling in the 
course of the experiment— to be also got rid of we might surmise that the agree- 
ment between the First Furrow and the other two planes would be considerable, 
provided that there were any reasons for supposing that the internal factors on 
which the direction of cell-division and the direction of differentiation depended, 
did themselves, under these circumstances, coincide. What these internal causes 
may be we shall have in a moment to enquire. 
Meanwhile there is still one point that demands some discussion. 
We have seen that the Symmetry of the Embryo — the position of the Sagittal 
Plane — follows very closely upon that of the previous Plane of Symmetry. It does 
not, however, absolutely follow, and, as we know, the correlation between the two 
