278 The American Naturalist. [Mareh, 
The egg is a specifically organized one-celled organism that develops 
epigenetically by process of multiplication of cells with subsequent dif- 
ferentiation. : 
Since each cell comes from the first (the egg) by division it likewise 
contains the beginnings of the whole and becomes differentiated and 
specific during process of development according to the position it oc- 
cupies in the whole at any period (gastrula, etc.). The reasons leading 
up to this position may be put under the following seven heads: 1. A 
complete organism may be formed from one of the first two or four 
cells.; accordingly in different cases cells of like origin must be put to 
forming different organs. 2. As the gastrula mouth may appear at 
various parts of the periphery the cells concerned must have different 
ates in different cases. 3. The same is true in the abnormal cases of 
formation of multiple gastrula mouths; then there may be formed four 
instead of two eyes, ears, ete. 4. Frog’s eggs that develop when held 
up-side-down must have the material utilized in a different way from 
the normal. 5. Thus also the triton larvee show various ways of using 
the similar cells when the first two are partly separated by a thread. 
6. When the frog develops up-side-down cases occur in which the lip 
of the blastopore is rolled outward and unites with the other lip so 
that the line of union is not between the edges of the lips but between 
the edge of one and the turning out surface of the other. Then the 
notochord and the medullary plate would be formed from cells quite 
other than those normally acting. 7. Changes in the cleavage process 
that so mix up the nuclear substance that it is assigned to different 
parts of the yolk in different eggs have no influence upon the normal 
result of development. 
Thus in place of the mosaic theory of Roux and the germ-plasm 
theory of Weismann we may substitute the theory of the controlling 
inter-adjustments of the embryonic cells and later of the tissues an 
organs. 
