398 The American Naturalist. [April, 
Experiments on Cleavage.’—Dr. Jacques Loeb, of the Uni- 
versity of Chicago, has madea most valuable addition to our know- 
ledge of the cleaving ovum. His experiments were simply the expo- 
sure of the eggs of sea urchins (Arbacia) to water containing more or 
less than the normal amount of sodium chloride. 
The general result is stated as follows: “If we reduce the irritabil- 
ity of the protoplasm of the egg by reducing the amount of water 
contained in it, the nucleus can segment without segmentation of the 
protoplasm. If we increase again later the amount of water, and - 
consequently the irritability of such an egg, the protoplasm at once 
divides into about as many cleavage cells as there are nuclei pre-formed. 
The segmentation of the protoplasm in the egg, and probably in every 
cell, is only the effect of a stimulus exercised as a rule by the nuclei.” 
- The following illustration of the character of the experiment is 
presented by the author: Eggs taken a few minutes after impregna- 
tion were divided into four lots, one put into normal sea water, one 
into that concentrated by adding 2 g. Na Cl per 100 ccm., the other 
two into sea water concentrated by addition of 1.3 g. NaC and 1 g. 
Na Cl per 100 cem. in each case. 
When the eggs in normal sea water had arrived at the two celled 
stage none of the others had as yet begun to cleave. In the least con- 
centrated solution the cleavage soon followed and in the more concen- 
trated solution it followed about an hour later, but in the most con- 
centrated solution no cleavage took place. Concentrations greater than 
2 g. per 100 ccm. produced plasmolysis. The form of the cells indi- 
cates the amount of water and the intracellular pressure; thus in 
normal water the first two cells are nearly hemispheres, but in concen-, 
trated solutions the cells approach more and more toward a spherical 
shape. 
Other experiments bring out the interesting point that the effect of 
salt is not to destroy but to suspend the cleavage phenomena. When 
the eggs are put back into normal water after staying some time, but 
not as long as twelve hours, in concentrated water, the suspended 
cleavage begins and goes on much as in a normally situated egg. The 
longer the eggs have been in the concentrated water the more numerous 
are the cleavage cells formed all at once when the egg is returned to 
normal water. An interval of about twenty minutes in the normal 
water must elapse before the sudden appearance of the retarded 
cleavage cells occurs. 
®Journal of Morphology, vii, 1892. 
