324 CAUSES OF VARIATION 



The first process seems to be to assume the characteristic 

 form, afterward to increase in size ; not only that, but regenera- 

 tion is possible in the entire absence of food, as will be seen 

 later, all of which indicates a more or less extensive transfor- 

 mation of material. 



Regeneration in embryos and eggs. 1 There is much reason to 

 believe that regeneration, especially by rearrangement, is more 

 pronounced in the embryonic than in the adult state. The frog 

 does not regenerate the leg, but the tadpole does. If the blastula 

 of the sea urchin be cut into two pieces, each will develop into a 

 perfect, but abnormally small, embryo. If the parts be separated 

 at the two- or the four-celled stage, each is capable of developing 

 into a perfect embryo, but at the eight-celled stage they are not 

 capable of such development. 



If each cell of the two-celled stage is capable of developing 

 into a complete individual, then the material at this stage must 

 be indifferent, that is undifferentiated. In other words, if the 

 first cleavage be considered as dividing into right and left halves, 

 and each half is capable of developing into a whole, the case is 

 exactly like that of regeneration in the split planarian ; if, how- 

 ever, the first segmentation be considered as dividing into ante- 

 rior and posterior regions, and if each may develop an individual, 

 it is exactly similar to the case of the regeneration of a worm 

 that has been cut in two crosswise into anterior and posterior 

 halves. In all cases some readjustment of material is involved. 



This separation is easily effected in the sea urchin by shaking, 

 in which case each part, below the eight-celled stage, develops an 

 individual. If in the frog the parts, even at the two-celled stage, 

 be separated, each collapses ; if instead one half be killed by a 

 needle, the uninjured part develops at first a half embryo, after- 

 wards making more or less successful "post generation" of a 

 whole embryo. 2 It is therefore a perfectly well-establrshed fact 

 that, in certain species at least, a part of an egg or embryo is 

 capable of developing into a perfect individual. 



To what extent this separation of segmenting eggs at the 

 two-celled stage may take place in nature is of course unknown, 



1 Morgan, Regeneration, p. 18; also chap, xi, pp. 216-241. 



2 Ibid. pp. 216-221. 



