ON THE RELATION BETWEEN THE SYMMETRY OF THE 
EGG AND THE SYMMETRY OF THE EMBRYO IN THE 
FROG {RANA TEMPORARIA). 
By J. W. JENKINSON, M.A., D.Sc, Oxford. 
PART I. 
As every embryologist will be aware, the relation between the first segmenta- 
tion furrow of the frog's egg and the sagittal plane of the frog embryo has been 
both the source of a famous theory and the central point of an equally celebrated 
controversy. For while the supposed coincidence of the two planes led Roux 
directly to the experiment in which one of the first two blastomeres being killed a 
half embryo was produced from the survivor and so to the definite formulation of 
the preformationist doctrine of " Selbstdifferenzirung " and " Mosaikarbeit," the 
criticism which this theory called forth was soon directed to a re-examination, and 
eventually resulted in a denial of the facts on which the hypothesis was based. 
Roux's own statement as to the relations between the planes in question, 
made in 1883, is sufficiently explicit. After describing the difficulties he had 
to encounter in obtaining an accurate measurement of the angle between the 
two, and giving the magnitudes of such measurements, unfortunately only a 
small number, as he was able to get, he concludes as follows : " So ist es wohl 
berechtigt wenn ich das hervorspringende Bestreben beider Ebenen zusaramen- 
fallen zu lassen, als das Gesetzmassige auffasse, und die gefundenen kleineren und 
grosseren Abweichungen nicht auf Abweichungen von dem Gesetz sondern auf die 
noch restirenden Fehlerquellen zuriickfuhre und so das Gesetz aufstelle. — Mit 
der Ebene der ersten Furchung wird beim Froschei zugleich auch die kiinftige 
Medianebene des Individuums bestimmt und zwar fallen beide zusammen " (Roux, 
1883, p. 109). Nor is the relation thus established one of mere coincidence, it is 
a causal relation, as we read in the Mosaikarbeit, published in 1893 (p. 850) : 
" Das Prinzip der organbildende Keimbezirke beginnt somit erst mit der Fur- 
chung eine 'feste' Bedeutung zu erhalteu ; dieselbe ist nicht bios eine topo- 
graphische, sondern auch eine causale," a conception which is of course a necessary 
