968 THE AMERICAN NATURALIST. [VOG XXXIII. 
according to the author, in any known chemical or physical force, but 
in an hypothetical Zernkraft, an elementary vital force which acts at 
definite distances from fixed points, like the active pole of the egg, 
or from the median-ventral line of the bilaterally symmetrical embryo, 
to determine in some definite manner the form. 
Various localization problems of a special nature are discussed. 
Treating of cleavage among the echinoderms, the author deduces 
from his own and others’ experiments upon the segmentation of egg 
fragments and of isolated blastomeres, the displacement of blasto- 
meres, etc., the conclusion that the protoplasm of the egg, in gen- 
eral, depends upon a certain factor, inherent in the protoplasm, upon 
which the cleavage and a general regulation of the egg mass depend, 
and he also asserts as proved the proposition that all parts of the 
cytoplasm during segmentation are respectively similar as regards 
their morphological “ prospective potency,” and that the nuclei are 
likewise similar each to each. From this standpoint he reasons that 
the earliest differentiation of the embryo is determined as to its posi- 
tion by a correlating force, this conclusion being quite opposed to 
the idea that the protoplasm of the egg consists of manifold elements 
arranged in some sort of a typical specific position, which bears a 
certain definite relation to later differentiation. If then the structure 
of the egg consists, as is assumed, merely of a polarity, namely the 
possession of a chief axis with unlike poles and one at right angles 
to it which also has unlike poles, how can one explain the localization 
of a variety of structures in the embryo not only at the poles but at 
any possible, though typical, position in the embryo? Likewise the 
arrangement of the bilateral groups of mesenchyme in the echinoids 
in definite typical positions, the appearance of the mouth in its proper 
place, independently of any possible contact stimulus proceeding from 
the archenteron, the localization of the ciliated band of Bipinnaria, 
all are problems of localization of a similar nature. 
The eggs of ctenophores and of mollusks naturally present- certain 
difficulties, since in therh no regulation of the entire egg mass is 
proved and a definite complex structure of formative materials 
within the ovum is conceivable. Nevertheless the author believes 
that the power is there, though dormant, and that a sort of preco- 
cious activity of the localizing force may explain the difference 
between these eggs and those of the echinoderms, Amphioxus, etc. 
Moreover, if a complicated structure is assumed for the ovum of the 
ctenophore or of the mollusk, then the very structure of such typical 
complex nature presupposes in oogenesis a localizing agency. 
