28 WHITMAN. [Vot. II. 
author, “the fertilized egg possesses absolutely no essential rela- 
tion to the later organization of the animal, no more than a snow- 
flake stands in any essential relation to the size and form of the 
avalanche which under certain conditions develops from it. 
That the germ always gives rise to the same form, is due to the 
fact that it is always brought under the same external condi- 
tions.” Immediately after the appearance of Pfliiger’s papers, 
it was pointed out by Agassiz and Whitman & that, “if gravita- 
tion were the sole guiding agency in cleavage, its effect ought 
to be zustantaneous, and it should be possible to change the 
direction of a cleavage-plane already in progress.” It was also 
shown that the /zme required to bring about a transposition 
of the third cleavage-plane, suggested a corresponding internal 
transposition of the active protoplasmic matrix of the ovum, tn- 
cluding of course the nucletz, “If a body constituted like the 
ovum is restrained by artificial means from taking its normal 
position, a redistribution of material must immediately set in and 
continue until the equilibrium is restored. The active portion 
of the ovum, having a lower specific gravity than the passive 
nutritive elements, would eventually recover its normal position, 
and thus the virtual axis of the ovum would inevitably right 
itself in spite of the inability of the ovum to rotate bodtly.” 
Later observations have fully verified these suggestions. 
As now maintained by Born,* Hertwig,® Weismann,® Kolliker,? 
and others, the cytoplasm alone is isotropic, while the nucleus is 
the seat of the directive and form-giving power in development. 
In this modified form, the doctrine of isotropy makes a much 
nearer approach to truth, but I believe that it is far from correct 
in its estimate of the functional importance of the cytoplasm. 
The logical consequences of this view are clearly presented 
by Oscar Hertwig® (p. 306) in the following words: “Az dite 
Kernsubstanz also sind die Krifte gebunden, durch welche die 
3 On the Development of some Pelagic Fish Eggs. Proc. Amer. Acad. Arts & 
Sciences, XX., p. 40, 1884. 
4 Biolog. Untersuch. Arch. f. mik. Anat., XXIV., 1885. 
5 Das Problem der Befruchtung und der Isotropie des Eies. Yenatsche Zeitschrift, 
XVIIL., 1885. 
§ Die Continuitat des Keimplasma’s als Grundlage einer Theorie der Vererbung. 
1885. 
7 Die Bedeutung der Zellenkerne fiir die Vorginge der Vererbung. Zedéschr. f. 
wiss. Zool.. XLII., 1885. 
