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THE POPULAR SCIENCE MONTHLY 



tion of substances and the differentiation of cells is not so evident in the 

 last named animals because these substances are not so strikingly colored. 

 Indeed the segregation and isolation of different protoplasmic substances 

 in different cleavage cells occurs during the cleavage of the egg in all 

 animals, though such differentiations are much more marked in some 

 cases than in others. 



ms. 



Fig. 31. Gastkula and Lakva of Styela, showing the cell lineage of various or- 

 gans, and the distribution of the different kinds of protoplasm to these organs. Muscle 

 cells are shaded by vertical lines, mesenchyme by horizontal lines, nervous system 

 and chorda by stiples. 



This same type of cell division, with equal division of the chromo- 

 somes and more or less unequal division of the cell bod} r , continues long 

 after the cleavage stages — indeed throughout the entire period of em- 

 bryonic development. Sometimes the division of the cell body is equal, 

 the daughter cells being alike ; sometimes it is unequal or differential — 



