206 Symmetry of Egg and Symmetry of Embryo in the Frog 
axis determined by the polar bodies in the same way as in the unaltered egg. 
Lillie has further succeeded, to some extent, in obtaining differentiation — the 
formation of the prototroch — though all division was entirely suppressed. We 
may add that cells which occupy an identical position in similar patterns of 
cleavage, may have, in different animals, a very diverse fate. 
In " typical " development, as Roux would call it, the symmetry of the egg, the 
symmetry of segmentation, the symmetry of the embryo may all be coincident. 
This is well seen in Annelids and Molluscs — where the D cell is posterior; in 
Ascidians — where the First Furrow is in the Plane of Symmetry of the egg and also 
in the Sagittal Plane of the embryo, the second transverse, the third horizontal ; 
again in the Cephalopoda — where the first is in the Symmetry Plane of the egg 
and in the Sagittal Plane, the second transverse ; in Ctenopliora — where the first 
and second divisions are respectively sagittal and transverse ; in the Sea-urchins, 
and in the undisturbed egg of the Frog. But that coincidence of segmentation 
with differentiation is not necessary for normal development is made abundantly 
clear by the experiments we have just considered, experiments which bring out 
the latent distinction between the two processes. 
We can only, it seems to me, conclude that there are present in the egg two 
sets of factors — those which cause a certain pattern of segmentation and those on 
which differentiation depends. The former must be sought for in the quantity of 
yolk in the egg (Balfour's rule), in the relation between the mitotic spindle and 
the cytoplasm (the rules of Hertwig), in the relation between the dividing 
centrosomes and the previous spindle axis (Sach's law) and in the capillary 
properties of the surfaces of the blastomeres. The latter on the other hand will be 
found — have, indeed, already been found — in certain necessary specific organ- 
forming substances of the cytoplasm. 
A limited part might indeed be assigned to mitosis in the determination of 
embryonic axes; the asters at the poles of the fertilization spindle might con- 
ceivably impress a bilateral symmetry upon the cytoplasm, — and it is as well to 
bear in mind that there is in the Frog a slight correlation between First Furrow 
and Sagittal Plane ; but speaking generally the two processes are very largely 
independent of one another. To the developing embryo the order in which its 
specific materials are cut up in segmentation is a matter of indifference — what is 
essential apparently is that the material should be subdivided until a definite 
quantitative relation is established between the dimensions of the nucleus and the 
dimensions of the cell (Boveri). 
Postscript. 
It is a very great pleasure to me to find that Professor Brachet, though working 
by different methods, has arrived at conclusions almost identical with mine. 
In his first paper {Arch, de Biol. xxi. 1905) he states that the angle between 
Plane of Symmetry and First Furrow may have any value. One hundred eggs 
