276 The American Naturalist. [March, 
With an omission of a critique upon Roux’s conceptions ot develop- 
mental processes we pass to the general conclusions that end the paper 
Pressure that changes the shape of the amphibian egg induces great 
changes in the directions and sequence of the cleavage planes and in 
the size of the cells. l 
The direction of the planes results from the form of the cell and the 
distribution of its protoplasm. : 
'Fhere is no causal connection between the first planes and the axes 
of the body ; the main axis of the body is not determined by the posi- 
tion of the first or second cleavage planes. 
In the various induced forms of cleavage the nuclei that are formed 
become, in the different cases, distributed to very various parts of the 
yolk; they may be vicariously distributed to all parts of the yolk, 
As the cleavage does not separate parts of the yolk predestined to 
form definite parts of the animal, so also the nuclei are not qualita- 
tively divided into different kinds of nuclear material for the various 
cells. Yet normal embryos with normally placed organs arise from 
such mixed up or unnaturally distributed nuclei. 
The egg contains no definite subst t apart to form special organs 
(liver-, skin-, retina-forming material) but it is isotropic. The contents 
of the egg ceases to be isotropic and becomes more and more specialized 
and organized in the process of cell multiplication with its important 
chemico-physical transformations (such as increase in the nuclear ma- 
terial 
In spite of this isotropy the egg is a definitely organized cell with 
_ yolk, protoplasm, etc., of different specific gravity. 
This specific nature of the egg contents and also the shape of the 
egg exercises a directive influence over the process of development; 
the embryo at first must be adapted to the form of the egg. 
The shape and position of the egg determine the position of the first 
cleavage planes. : 
As no rearrangement of heavy and light portions takes place In 
cleavage the distribution of mass in the egg corresponds to that in the 
blastula. : 
When the walls of the blastula are not uniform the gastrulation can 
take place only in a special zone which is below the equator when there 
is less yolk, as in the amphibian egg, and above when there is very 
much yolk, as in meroblastic eggs. 
From an oval or elongated egg there is formed an elongated blas- 
tula, gastrula, ete. (in triton and insects, ete.). 
