1102 The American Naturalist. [December, 
cells in the sixteen cell stage, after the egg membrane had been 
removed. At this stage there are eight equal cells, of the vegetative 
region, four large cells and four micromeres of the animal region. 
Complete pleuti arise when the micromeres are absent ; when only 
some of the above 8 negative cells are present. 
The facts in connection with the author’s previous experiments upon 
compressed eggs show, he thinks, that the nuclei of a cleaving egg are 
equivalent and that the germ layers are not separated during cleavage 
by any qualitative nuclear divisions. 
The presence of micromeres is shown to be unnecessary for the for- 
mation of an echinus gastrula. 
(Some attempts to form fusions of cleaving eggs are mentioned here, 
though they were not successful; the eggs adhered (after treatment 
with ehloroform) but eventually sopari again). 
The last chapter deals with some fundamental questions and results 
reached by this mode of investigation. 
The outcome of all the author’s work isin opposition to the views 
held by Roux and Weismann, for it is shown that the cleaving egg, at 
least in these echinoderms, is not differentiated as regards its nuclei 
which are all alike in quality. Not so the protoplasm of the eggs 
which the author maintains must be anisotropic from the first, must 
possess a differentiation in direction so that all of it, or a part of it 
when removed, acts like a stimulus in producing the first differentia- 
tion of form, which is the first difference between the ends of the main 
axis of the larvæ. When the larva becomes bilateral this is due to a 
like direct specialization in the protoplasm, finding its expression in the 
ifference seen between the dorsal and ventral sides of the larvæ. 
- Some organs may owe their position directly to this determined axial 
condition, others may arise from more indirect causes, from stimul- 
Ms come either from other separate organs or from wines the organi 
Cells become this or that according to their position; the organs 
arise as functions of the positions of the cells forming these organs. 
Thus is the peculiar half-dead embryo Roux obtained in the case of 
the frog; the cells are forced to remain in their original or normal 
positions by the presence of the dead half-embryo ; if this were away 
the cells could rearrange themselves and so des into new positions 
would form one complete embryo, not a half embryo. 
The evolution of form in an individual is the result of stimuli and 
reactions of a binc interacting nature. These only are investigated 
in “ gs studien.” The primal SM of form 
