WEISMANN'S THEORY OF HEREDITY 51 



further that the different kinds of cells contain different de- 

 terminers and consequently that as the egg divides up into 

 cells which form the different parts of the body, these cells 

 must receive different determiners. But microscopic exami- 

 nation of the cells of the body reveals no such differences; it 

 shows differences in pretty much everything except chromo- 

 somes, which remain remarkably constant. 



Boveri (1887) has described one case which seems to 

 support the idea that changes in the chromatin occur, as 

 body-cells become distinguishable from germ-cells. In the 

 parasitic worm, Ascaris, the chromosomes are seen partially 

 to break up and disintegrate in those cells of the embryo from 

 which the body arises, whereas the original ovarian structure 

 remains unmodified in the germ-cells. No similar case, how- 

 ever, has been described in other organisms, so that it seems 

 very doubtful whether the observed changes have the signifi- 

 cance originally attached to them by Boveri.^ There are good 

 reasons for believing that the chromatin content of each cell 

 of the body is like that of every other cell of the same body, 

 and that differentiation results either (a) from the position 

 of a cell in relation to other cells, which will accordingly regu- 

 late its intake and output, or (6) from an original difference 

 in substance contained in the cytoplasm of the cell (the extra- 

 nuclear part). Such cytoplasmic differences between cells 

 arise, during development, from the fact that the egg cyto- 

 plasm, at the beginning of development, is not homogeneous, 

 and consequently the cells into which the egg divides are not 

 alike in cytoplasmic content. 



2. Regeneration. A man who loses a leg or an arm is de- 

 prived of the same for the remainder of his life, but many of 

 the lower animals can restore lost parts by a process which 

 we call regeneration. If a young salamander, a crab or a 

 lobster is deprived of a leg, a new leg grows out again from 



1 It is true that Hegner (1914), confirming Kahle (1908), has also observed 

 "diminution of chromatin" occurring in the differentiation of somatic cells in an 

 insect, Miastor, but in numerous other animals studied by Hegner he has found 

 no such diminution of chromatin but has observed the germ-cells to be differen- 

 tiated solely by cytoplasmic changes. 



