RELATIVE STABILITY OF LIVING MATTER 



343 



or a whole half-sized dwarf might be formed, according to the 

 position of the blastomere." l 



Causes of differentiation. These facts show : 



1. That a primary cause of differentiation lies in the polarity 

 of the ovum. 



2. That this polarity is due primarily to gravity separating 

 its heavier from its lighter parts. 



3. That the egg is at first, in many cases at least, indifferent, 

 and that its polarity may even be changed after having once 

 been well established. 



4. That if the segmenting ovum may be separated into its 

 blastomeres, and each may produce a complete individual (Am- 

 phioxus, etc.), 2 then the egg as a whole is not only totipotent 

 but its earlier segments as well are each capable of producing 

 all the parts of the body. 



5 . The polarity and the development of a part are influenced 

 largely by its position with reference to other and especially 

 larger parts. 



Roux found that when, in the two-celled stage of the frog's egg, 

 one of the blastomeres was killed by a hot needle, the remaining 

 one developed a half embryo, 3 whereas in many cases a single 

 blastomere is known to be entirely capable of producing a whole 

 embryo if freed from its neighbors. 



Again, a section from an earthworm tends to regenerate 

 head matter at its anterior end and tail matter at its posterior 

 end ; but if a small piece be grafted, even in a reversed position, 

 on the anterior end of a larger piece, it will regenerate head 

 matter from its posterior extremity, showing how the polarity of 

 the smaller piece is, so to speak, overcome by that of the larger. 4 



A consideration of these facts leads Wilson to quote Hertwig 

 as follows : The relative position of a blastomere in the whole 

 {assemblage'} determines in general what develops from it; if its 

 position be changed, it gives rise to something different ; in other 

 words, its prospective value is a function of its position?* 



1 The different cells of the earlier cleavages (the 2-, 4-, 8-, i6-celled stages, etc.) 

 are known as blastomeres. 



"~ Wilson, The Cell, p. 423. 4 Morgan, Regeneration, p. 8. 



3 Ibid. p. 380. 5 Wilson, The Cell, p. 41 5- 



