EXPERIMENTAL ZOOLOGY 185 



have been the experiments and their results that I can mention 

 only the gist of our conclusions upon the question of the 

 extent to which adult organization is foreshadowed within 

 the egg, upon the extent to which the egg is preformed. 



Some eggs appear to have within them definite recog- 

 nizable substances, unlike the adult parts but from which 

 the adult parts take origin, and are to that extent thus early 

 differentiated or preformed. In others, this differentiation 

 is not apparent and seemingly one area is of the same value 

 as any other. We conclude, therefore, that the eggs of dif- 

 ferent animals are not all alike with respect to their dif- 

 ferentiation at the one-cell stage ; that the differentiation, while 

 present in some cases at an early stage, does not appear in 

 others until they are considerably advanced in their develop- 

 ment; while, in those forms which have as adults great 

 capacity for the regeneration of lost parts, the organism ap- 

 pears as though never so far differentiated as to be unable 

 to reform itself from even a minute part of the whole. 



In problems of this nature, satisfactory analysis can 

 only be based upon experiments which subject the organism 

 to new and controlled conditions, and no amount of observa- 

 tion upon normal development will go so far toward answering 

 the question, whether at the two-cell stage we have the right 

 and left portions of the animal irrevocably distributed to 

 right and left cells, as will the simple experiment of shaking 

 these two cells apart and seeing what happens. And in a 

 simple way, this illustrates the whole point of the work in 

 experimental embr\'olog>'. Unfortunately, we can only take 

 our material as we find it, and we are often limited in the 

 experiments by external conditions and by the structure of 

 the living things themselves. Still, astonishing ingenuity is 

 often seen in surmounting such difficulties, and experimental 

 embryology presents to us one of the most fruitful fields now 

 open to the zoologist. 



