1913.] NATURAL SCIENCES OF PHILADELPHIA. 19 



On the other hand, the variation may be a response to change of 

 environment, the animals becoming adapted to arid conditions. 

 Of course, this adaptation is evident as regards the reduction in size, 

 but it is not yet known that animals living with deficiency of moisture 

 necessarily become more or differently sculptured. The chief 

 effect of this aridity would be to reduce the moisture of the body, 

 and this may conceivably produce an increase of sculpture. Re- 

 duction of moisture followed by an increase of moisture tends to induce 

 proliferation. This has been noted by Loeb^ in the case of the eggs 

 of the sea urchin. When newly fertilized eggs are placed in hyper- 

 tonic sea water for three or four hours and then brought back into 

 normal sea water, they divide into from six to sixteen cells in about 

 ten minutes, and in some cases even into about forty cells inside 

 of twenty minutes. The reduction of moisture in the egg was here 

 sufficient to inhibit cell division, but not enough to prevent nuclear 

 division. When put back into normal sea water, a most powerful 

 streaming of the protoplasm was observect. This streaming seemed 

 to occur around the chromosomes and fragments of nuclear matter 

 as centres. At length each knob or projection formed by the 

 streaming became a separate cell. The effect of the hypertonic 

 sea water (made by adding salt to normal sea water) was to with- 

 draw water from the cell. Putting the egg back into normal sea 

 water added water to the cell. Estivation must result in with- 

 drawal of water from the protoplasm, and may proceed to the point 

 of gelation. Addition of water to the protoplasm after such aesti- 

 vation, might readily result in proliferation as a result of the irregular 

 nuclear division produced during the aestivation period when cell 

 division could not occur. The irregular thickening of the shell 

 that forms the peripheral tubercles and other increase of sculpture 

 is of the nature of a proliferation of the shell. This is conceivably 

 due to the irregular cell division produced by aestivation, which in 

 turn is preceded by nuclear division without accompanying cell 

 formation during the aestivation period. Normally a cell dividing 

 produces two daughter cells, but under this fluctuation of the water 

 content the number of daughter cells may be from six, eight, etc.; 

 or M = 2D may become M = 4D (or 6D or 8D, etc.). 



Boveri^ has shown that the conditions which bring about cell 

 division seem to depend upon a ratio between the mass of the 

 chromosomes to the mass of protoplasm being established, and occurs 



' Loeb, Jour, of Morphology, Vol. 7, p. 253, 1892. 

 8 Boveri, Zellen Studien, Heft 5, Jena, 190.5. 



