272 The American Naturalist. ; [March, 
EMBRYOLOGY. 
Cleavage and the Formation of Organs.—An important ad- 
dition to the accumulations of experimental embryology has been re- 
cently made by Oscar Hertwig’ in the hope of clearing up the fogs 
that envelop the important subject of the relations of the cells of a 
cleaving ovum to the subsequently formed organs of the adult. 
While His, Roux and Weismann have seen in the ovum or germ a 
preformation of parts or organs and looked upon the cleavage cells as 
different in quality from the first, regarding the process of embryo for- 
mation as an evolution (in the old sense), Driesch and Hertwig, from 
experimental studies, now regard the ovum as isotropic, its first cells are 
qualitively alike, the embryology is an epiginetic formation of organs. 
The process is one of inter-relation of the cleavage cells. ; 
In the present paper the author describes a Jong series of experi- 
ments made upon frogs’ eggs and applies them to the overthrow of 
Roux’s main position, meeting that investigator upon his own grounds. 
The methods used are: the compressing of the eggs between glass 
slides placed horizontally, vertically or inclined; the com pressing of 
the eggs by drawing them into narrow glass tubes placed horizontally 
or vertically ; the partial separation of the first two cleavage cells (in 
the Triton) by means of a loop of fibre from a cocoon tied about the 
the egg; the injury of one of the first two cells by the insertion of a 
needle; and the same result by the use of an electric current, continu- 
ous or interrupted. oa 
We will first give some of the chief facts obtained by each meth 
. and then the author’s conclusions, : 
When the eggs lie in the normal position upon a glass slide but are 
compressed by the slide that rests upon them so as to be no ~~ 
spherical but considerably flattened, the main axis from the black tot 
light pole being thus made the shorter, by a third or a fourth, the pal 
cleave in an abnormal manner.. The third plane is not horizontal a 
more nearly vertical so that the first eight cells form a bilaterally < i 
metrical set of four on each side the second cleavage plane. me 
the pressure is exerted upon the sides of the egg, which is done by p 
; be 
* 3 a 
_ ‘Edited by E. A. Andrews, Baltimore, Md., to whom communications mY 
se 
nt. 
2Archiv fr Mik. Anatomie. 42. 29 Dezember, 1893, pps. 662-794, sda 
