DYNAMICS OF MORPHOGENESIS 161 



embiyos, including those of the amphibia, and it can undoubtedly 

 be used for many other forms. With the higher organisms the 

 chief difficulty connected with its use lies in the determination of 

 the time of death. If the time of death in such forms or in their 

 parts can be determined by any other simple method than that 

 of disintegration, there is no apparent reason why it should not be 

 possible to compare rates of reaction by determining the physio- 

 logical resistance to certain reagents of such forms or their pafts. 

 In the following sections the practical technique of the two 

 methods is described. 



4. The technique of the direct method 



In my own experiments it has proved most convenient to use 

 lots of ten animals or pieces for each test. When larger numbers 

 than ten are used the examination of a lot often requires too 

 much time so that it is difficult to avoid falling behind in keeping 

 the records. In many cases the comparison of single individuals 

 or pieces gives perfectly definite and constant results, but the 

 use of the larger number obviates the necessity of frequent repeti- 

 tion and also permits slight differences to appear which might not 

 be discovered in the comparison of single individuals. 



In the case of whole animals all ten of one lot are taken from 

 the same stock, i.e., they have been kept in the same vessel, have 

 received the same food and have been subjected to the same exter- 

 nal conditions. Moreover, worms of as nearly as possible the 

 same size are selected. In the case of pieces the ten of a lot are 

 from animals of the same stock and the same size and the pieces 

 are as nearly as possible of the same length and from the same 

 region of the body. Every one of these factors is important for 

 the result and in order to obtain definite results it is absolutely 

 necessary that the material be standardized in this way. 



When the animals are selected or the pieces cut they are usually 

 placed as nearly as possible simultaneously or at like intervals in 

 the required concentration of the reagent used : in certain experi- 

 ments the animals must be placed in the reagent and the pieces 

 cut in it. I have found Erlenmeyer flasks convenient except 

 where the animals or pieces are so small that the use of a compound 



