2i6 THE PHYSIOLOGY OF TWINNING 



or a senescent condition and in no sense a reversion to an 

 ancestral condition. Therefore it is a mistake to consider 

 that twinning in any sense represents the now generally 

 lost asexual phase in the life- cycle of the vertebrates. 



Gemmill (191 2) is doubtless the originator of this 

 mistake as may be judged from the following passage : 



The view has often been suggested that the blastoderm may 

 be looked upon as a stock, able to give rise vegetatively, so to 

 speak, to more than one embryo. The natural comparisons have 

 been drawn between this faculty and the alternation of generations 

 which occurs normally in some groups of lower animals and in 

 plants. It has been sought to recognize alternation of generations 

 in the development of all animals. More probably, however, in 

 animals, twinning, double and triple monstrosity, polyembryony, 

 and alternation of generations, provide instances in which a common 

 "potentiality" has become realized, and beyond that are not neces- 

 sarily connected by any nexus of a direct or phylogenetic character. 



This is at least a non-committal statement of this 

 point of view, but it has evidently led to a considerably 

 less cautious statement by Stockard, who says: 



The suggestion has frequently been made that the blastoderm 

 may be looked upon as a stock able to give rise asexually to more 

 than one embryo. Since the natural process of budding to form 

 four or more embryos in the armadillo is recognized, and accessory 

 individuals may be produced experimentally from other vertebrate 

 eggs, it becomes evident that even man and the highest animals 

 may actually at times exhibit an alternation of the sexual and 

 asexual processes of reproduction. 



If any real significance is to be attributed to finding 

 among vertebrates a similitude of the so-called alterna- 

 tion of generations of the lower forms, one need hardly 

 go so far afield to discover it. Instead of tracing it 

 back to a typically sessile and radial group such as the 

 Coelenterates, we can find it among the Rhabdocoels, 



