160 The American Naturalist. [February, 
eggs to mechanical pressure by the very simple expedient of allowing 
a cover-glass to rest upon them with more or less force as controlled by 
` a hair inserted under the glass. 
Eggs flattened out to several times their original diameter do not 
Jose their power to develop, even when the egg membrane is ruptured. 
If the eggs are fertilized and undergo cleavage under pressure the 
resulting cells are not arranged in the form of a sphere but form a flat 
isk. This arrangement of the cells results from the arrangement of 
the nuclear spindles. While in a normal egg the spindles of the first 
four cells would stand vertically, as it were, and produce a new set of 
cells by a horizontal cleavage, under pressure the spindles are found 
all lying at right angles to the compressing force; that is, parallel to 
the cover glass or horizontally, and hence give rise to cells all lying in 
one plane. When the pressure is removed the cells remain at first in 
a flat disk, though each cell becomes rounded. Subsequent divisions 
of these cells give rise to a spherical mass capable of forming a com- 
plete normal embryo. 
With the aid of figures the author makes it clear that in such cases 
material which would normally have taken part in the formation of 
only one pole or side of the embryo must now contribute to opposite 
poles or sides. In the sea-urchin there is thus no early specialization 
or differentiation of the material found in the cleavage cells; the mix- 
ing up of the blastomeres would not prevent the formation of a com- 
plete normal animal: the author concludes they could be mixed in all 
_ possible ways without destroying the normal symmetry of the adult. 
In the philosophical discussion that takes up the final portion of the 
paper, the author considers the methods of morphological research and 
upholds the experimental method, which tends to promise the redut- 
tion of Biology to a scientific basis sooner than the descriptive, histor- 
ical or mechanical lines of inquiry possibly can. 
Studies in Insect Embryology.—Dr. H. Henking* having pře- 
viously published extensive researches upon the formation, maturation 
and union of the sexual cells in Pieris brassica and Pyrrhocoris apterus 
concludes this comparative study with additional facts upon the eB o 
of the latter insect as well as those of Agelastica ulni, Donacia lam- 
pyris, Tenebrio, Lasius, Rhodites, Bombyx, Musca, and other represen- 
tatives of nearly all the chief groups of insects and then summarizes 
the whole with the aid of most interesting comparisons and sugges- | 
. 
tions. 
* Zeit. f. Wiss. Zool., 54, 1892; 51, 1891; 49, 1890. 
