TWINNING AS A MODE OF REPRODUCTION 215 



a local breaking away or emancipation of parts occurs, 

 and the isolated parts become the beginnings of new 

 organisms. If the integrative forces are moderately 

 lessened or inhibited, only certain less completely inte- 

 grated or outlying parts tend to gain physiological 

 independence. Functioning independently tends further 

 to isolate and, in the end, the part becomes not only 

 physiologically, but physically independent. Then 

 follows the process of reconstituting the specific form, 

 and this is able to take place whether the isolated part 

 is a single germ cell, a small mass of internal cells, a 

 lateral bud, the isolated halves of a bilateral primordium, 

 or the posterior half of an axiate animal. 



TWINNING AND ALTERNATION OF GENERATIONS 



Among the lower animals it is very common to find 

 that the life-cycle is much more complex than in the 

 higher animals. In the colonial hydroids, for example, 

 an egg develops a young hydranth which by lateral 

 budding produces a whole series of similar asexual 

 hydranths. Late in the season certain budded individ- 

 uals, medusa buds, develop into free-swimming sexual in- 

 dividuals, medusae, and these produce eggs and sperm, 

 which in turn unite to give us the fertilized egg again. 

 We thus seem to have an alternation between an asexual 

 mode of reproduction and a sexual, which has been called 

 alternation of generations. Without seriously opposing 

 the value of thus marking off ^ what is essentially a 

 continuous ontogeny into separate generations, I wish 

 to enter a protest against what appears to me to be a 

 misuse of the facts of twinning, especially that in the 

 armadillos. Twinning is, in my opinion, a cenogenetic 



