324 STUDIES IN GENERAL PHYSIOLOGY 
reservedly employed, as it is questionable whether one of the 
cells of the eight- or sixteen-cell stage can really be con- 
sidered as identical with the eighth or sixteenth part of 
the unsegmented egg. It is possible that through the 
process of segmentation the substance of the egg is sepa- 
rated into unhomogeneous parts. It is further possible 
that the metabolism during segmentation changes the 
material contained in the various cleavage cells unequally. 
This might result, for example, in this, that one of the cells 
of the eight-cell stage would no longer be able to develop 
into a complete embryo, while one-eighth of the same egg 
before cleavage might have been able to form a complete 
embryo. A short time ago I published a method which 
enables us to divide the unsegmented fertilized egg into small 
pieces capable of development.’ The method consists in 
bringing the sea-urchin eggs, after fertilization, into sea- 
water which has been diluted through the addition of 100 
per cent. of its volume of distilled water. The contents 
of the egg absorb water rapidly, and the thin egg-membrane 
ruptures at one or more points; a portion of the proto- 
plasm flows out of these ruptures, which assumes a spheri- 
cal form, and usually remains connected with the egg. 
(Fig. 74.) When the eggs are returned to normal sea- 
water, they begin to segment, the extraovate as well as the 
protoplasm which has remained in the egg form separate 
blastule, and twins result from the egg. These may 
either remain attached to each other or separate later on. 
The latter usually occurs. It is possible, however, that 
not only two but several protoplasmic drops may exude 
from the egg, and in this way more than two embryos may 
develop from one egg. Finally, in some cases where only 
one extraovate exists, a separation of groups of cells may 
occur during segmentation which leads to the production of 
1 Pfliigers Archiv, Vol. LV; and Biological Lectures Detivered at Woods Hole 
1893 (Boston: Ginn & Co.). 
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