The Causes of Twinnino' 



371 



another plant jjhenomenon that might 

 be compared with twinning in animals. 



Intensive study of twinning would 

 seem justified by the recognized inter- 

 est of human twins as our most famil- 

 iar evidence of the power of heredity 

 to determine all of the characters in 

 advance of development. The facts 

 show that twinning is not a simple 

 phenomenon, or merely an extra di- 

 vision of the fertilized egg cell, but 

 involves a change of developmental 

 behavior in groups of cells. The 

 groups not only subdivide and separate 

 but regenerate. The program of devel- 

 opment is reorganized in relation to 

 new median lines or planes of sym- 

 metry, so that two individuals are de- 

 veloped. As a result of twinning, 

 cells from the median parts of the un- 

 divided group must become lateral, 

 or regenerate lateral cells. 



Totipotence, or genetic equality of 

 the embryonic cells has, of course, to 

 be assumed to account for the fact 

 that the developmental functions of 

 many of the cells are changed to 

 accord with the revised plan of pro- 

 ducing two individuals instead of one. 

 The recognition of totipotence is an 

 admission of the fact that the courses 

 of development can be changed, but 

 totipotence alone does not account for 

 changing the plan, or for the assign- 

 ment of the new and highly coor- 

 dinated parts that the cells must play 

 in the drama of development. Char- 

 acters are expressed in all of the 

 stages of development, in the sequence 

 that is proper to the successive stages. 

 Definite dififerences may appear in 

 juvenile or embryonic stages, as well 

 as in the adult stage if expression is 

 varied or abnormal. 



Though the characters of the adult 

 are determined by the behavior of the 

 cells in development, yet the cell be- 

 havior itself apparently is determined 

 by a form of control which has the 

 power to regenerate and reorganize, 

 which amounts to redetermining the 

 expression of the characters. No 

 idea of such a directing or determining 

 agency is afforded by current concep- 

 tions of heredity as a complex of in- 

 dependent Mendelizing "characters." 

 The evidence that such changes of de- 

 velopment as are involved in twin- 

 ning can be initiated or induced by 

 environmental causes, while the same 

 ])lan or pattern of development is car- 

 ried out with such exactness as twins 

 often show, should be of interest to 

 every student of heredity. 



For some purposes twinning may be 

 considered as a very slight form of 

 abnormality, though connected by 

 scarcely perceptible degrees with many 

 more definite abnormalities. The forms 

 of control and the nature of the ad- 

 justments that are required for the 

 l^rocesses of normal heredity are 

 shown most effectively in contrast with 

 defects or disturbances of the normal 

 course of development. The ability 

 to resist adverse conditions of the en- 

 vironment and maintain a normal 

 course of development is an important 

 "character," or aspect of heredity. 

 The best reason for the study of ab- 

 normal forms is to gain a better un- 

 derstanding of normal heredity. Mon- 

 strosities and malformations have been 

 recorded extensively, but little use can 

 be made of such facts for purposes 

 of instruction until they are somewhat 

 organized and interpreted, as many are 

 in the present work. 



O. F. Cook. 



