STRUCTURE AND DEVELOPMENTAL RATE 163 



gives rise to a partial and not an entire embryo. Such eggs have 

 an early differentiation and localization of 'organ-forming stuffs' 

 and these stuffs are unequally distributed to the blastomeres even 

 at the first cleavage. The individual blastomeres are, there- 

 fore, not totipotent, but only capable in their later development 

 of giving rise to certain parts of the embryo and not the whole. 

 The eggs of a number of worms and molluscs present this very 

 early localization of differential stuffs, yet in some of these vari- 

 ous types of double individuals are not uncommon. These 

 double individuals I believe, in the light of evidence contained 

 in the literature along with that presented here, are the results 

 of a simple process of budding. 



Again it was shown by Enders and later by Spemann ('03) 

 that double specimens not only resulted from the separation of 

 blastomeres, but the late blastular and gastrular stages could 

 mechanically be caused to develop into double instead of single 

 individuals. The degree of duplicity depended somewhat upon 

 the extent to which the eggs were constricted in a given plane. 

 This was evidently a case of dividing or separating into two 

 parts the growing region of a single individual and thereby estab- 

 lishing two new growing points instead of the original one. The 

 division of a single growing bud into two may be illustrated on 

 plant buds, embryonic animal limb buds, etc. The interpreta- 

 tion of the two separated regions as being the exact derivatives 

 of the two original blastomeres, as Wilder has suggested, is in 

 many cases entirely implausible. 



Doubleness in nature is probably due to a modification of a 

 budding process, and double monsters and actually identical 

 twins, like all other abnormalities, may result from an arrest or 

 inhibition in development. To state that twins and double indi- 

 viduals are induced by a developmental arrest seems at first 

 thought almost absurd; for how could an arrest serve to give a 

 formation structurally exceeding the normal in extent? One 

 might accept developmental arrests as explanations for many 

 deficiencies in structural expression, but such an explanation of 

 excessive conditions or double-headed and twin individuals. would 

 scarcely be suggested. In the present consideration, however, it 



