438 SOME GENERAL PROBLEMS OF DEVELOPMENT 



differentiation that can be recognized at present. This is true 

 of the frog's egg before the entrance of the spermatozoon. In 

 eggs of the latter sort, the vakies of the various areas are more 

 nearly equal, and recognizable differentiation appears only at a 

 subsequent stage of development. The truth seems to be that 

 the eggs of different animals are not alike with respect to their 

 visible differentiation at the one-cell stage; that the first signs of 

 differentiation, while visible in some animals at the one-cell stage, 

 are not apparent in others until a later stage of development; 

 while in those forms that have, as adults, great capacity for the 

 regeneration of lost parts, the organism is never so completely 

 differentiated as to be unable to reform an entire body from a 

 portion of the whole. Some kind of organization, or " prefor- 

 mation " in the modern sense, must be present in every egg, 

 whether it is ^asible or not. Otherwise there can be no explanation 

 of the phenomena of heredity that can satisfy the demands of sci- 

 ence. The embryologist is of necessity a preformationist, although 

 not in the older sense of the word. The situation is analogous to 

 that found in Physics and Chemistry, in which the existence of 

 invisible molecules and atoms is postulated as a basis for the 

 visible phenomena. In problems of this nature, satisfactory 

 analysis can only be based upon experiments which subject the 

 organism to new and controlled conditions. No observation of 

 normal development, however extensive, will go so far toward 

 determining whether at the two-cell stage the right and left por- 

 tions of the animal are irrevocably distributed to right and left 

 cells, as will the simple experiment of separating these two cells and 

 seeing what happens. 



In addition to this study of embryonic stages, there is another 

 method of attacking the problem of preformation, which may be 

 illustrated as follows: If, in a game of cards, the hands are dealt 

 out and the cards in each hand then examined, it is possible, if the 

 nature of the dealing is known, to infer the manner in which the 

 cards were arranged in the pack before the dealing began. One 

 must either recognize a causal relation between the arrangement 

 within the pack and the arrangement that appears as a result of 

 the deal, or must assume the miraculous origin of the latter. Study- 

 ing the inheritance of the qualities that appear in an adult animal 

 is like examining the cards in a hand, while knowing something 

 of the dealing, without knowing the organization of the pack. 



