324 STUDIES IN GENEEAL 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. 1 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 

 blastulse, 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 



i Pflilgers Archiv, Vol. LV; and Biological Lectures Delivered at Woods Hole. 

 1893 (Boston: Ginn & Co.). 



