NATURE, SCOPE, AND CAUSES OF TWINNING 5 



ism between the mode of cleavage and the incidence of 

 twinning is confirmed. 



THE CAUSES OF TWINNING 



For a long time I have held the view that twinning 

 is essentially a phenomenon involving a physiological 

 isolation of equivalent parts of the blastoderm and a 

 regulation of the isolated or twinned regions into complete 

 embryos. As to the cause of physiological isolation, 

 I have maintained, for the armadillo at least, that 

 the essential feature is temporary cessation or radical 

 retardation of development at a critical period. It is 

 not especially important for the theory to determine the 

 exact cause or causes of the lowered developmental 

 rate, for we know that a great many agencies give the 

 same end-result so long as they accomplish a retardation. 

 What retardation evidently does to the egg or embryo 

 is more or less completely to disorganize its integrational 

 relations, or, in a word, to deaxiate it. If through 

 retardation the polarity of the egg or embryo becomes 

 so weakened that there are no relatively high or low 

 metabolic regions, all parts of the embryo are left on a 

 parity; so that, when normal developmental conditions 

 once more appear, any part of the embryo may become 

 the apical point and a new gradient will be established. 

 What usually happens, however, is that enough of the 

 original axis of polarity persists to permit just two 

 equivalent points on the axis, at equal distances from the 

 original apical point, to remain in such a condition that 

 they are favored when developmental conditions return, 

 and these become the growing points or apical ends of 

 twin embryos. Very commonly the developmental re- 



