1894.] Embryology. 277 
` As many eggs have elso a bilateral arrangement of their component 
substances there must follow a bilateral blastula in which the place for 
formation of the blastopore will be more sharply defined. 
The chief axes of the embryo may correspond approximately to the 
first cleavage planes in eggs that are bilaterally symmetrical or that 
have one long diameter, since the character of the egg determines 
both. 
In the gastrulation of the amphibian egg there is a revolution about 
an axis cutting the plane of symmetry and the plane of equilibrium. 
Eggs of complex consistency are acted upon by gravitation so that 
they are oriented and if bilaterally symmetrical stand with the plane 
of symmetry vertical since this is also the plane of equilibrium. 
If such eggs are forced to develop in a constrained position they form 
asymmetrical embryos so that gravitation is, in a sense, one of the in- 
fluences determining structure. 
If one of the first two cells of the egg is destroyed the other devel- 
ops into a tolerably normal embryo having, however, some of its less 
important regions defective. 
When one cell is but partly destroyed it may later form cells that 
are added to the uninjured half to help form the embryo. This secon- 
dary formation of cells in the injured half may be from the uninjured 
nucleus of that cell, or sometimes, by the migration of nuclei from the 
uninjured egg-half into the injured egg-half. 
The development of the uninjured half, by itself or with the aid of 
part of the injured half, follows the same laws as the natural ontogeny 
of the species. 
The injured yolk acts in the development of this half of the egg as the ” 
nutrient material does toward the formaitive in a meroblastic egg. 
The process of postgeneration described by Roux does not take 
place nor is there a revivification of the destroyed egg-half. 
Embryos with cleft blastopore cannot form double monsters by the 
process of postgeneration that Roux brought in to explain such a 
formation. 
We cannot form at will half-anterior, -posterior or -lateral blastulas 
or embryos by destroying one of the first two cleavages cells. 
: In these cases of injury complex processes of adjustment may result 
in the formation of a normal embryo under changed circumstances. 
: The results obtained by these pressure experiments as well as the 
ery to one of the cleavage cells demonstrate the untenability of the 
Mosaic theory, the theory of specialized germ areas and Weismann’s 
theory of germ plasm. 
