RELATIVE STABILITY OF LIVING MATTER 339 



It involves no breach of continuity, and is but a continuation of 

 the vital processes going on in the parental body. What gives 

 development its marvelous character is the rapidity with which 

 it proceeds, and the diversity of the results attained in a span 

 so brief." 



We can define the chief mystery of development as lying in 

 the facts of differentiation and definite termination to growth, but 

 if we ask how the impulses governing such complicated results lie 

 latent in a single cell, and how they operate in orderly sequence 

 beginning and ending at the proper moment, we have asked the 

 " final question," and " in approaching it," says Wilson, " we may 

 as well make a frank confession of ignorance." 



About all we can hope to do in the present state of knowledge 

 is to throw light upon the question of comparative stability of 

 organized matter and note the conditions under which one kind 

 of cell gives rise to another of a different order. On this point 

 certain facts of development have an important bearing. 



Cell division. All differentiation during development is by 

 cleavage of the germ cell. The phenomena of division and mito- 

 sis are in general the same in the cleavage of the germ cell and 

 the development of the embryo as in the division of ordinary 

 tissue cells, except that the body cells for the most part give 

 rise only to others like themselves, while those of the germ and 

 the embryo give rise not only to others like themselves but to 

 many different kinds as well. 



This distinction is relative rather than absolute, for we remem- 

 ber that in certain species a small part, even a leaf, is able by 

 regeneration to give rise to a new individual, involving differen- 

 tiation similar if not equal to that of embryonic development. 

 However, in many species and with most tissues the power of 

 typical differentiation appears to be lost in the first development. 

 In this connection it is well to remind ourselves that in many 

 species the amount of chromatin matter in the somatic cells is 

 different from that in the germ cells. 1 



Geometrical character of cleavage. The earliest cleavage of 

 the germ cell is governed by two definite geometric principles 

 announced by Sachs : (i) the cell typically tends to divide into 



1 Wilson, The Cell, p. 426 (concerning Boveri's investigations on Ascaris). 



