DIFFERENTIATION AND REJUVENATION 157 



determined so that the precise length of a fibre can 

 be measured, and its diameter also. Hence the total 

 volume of a fibre may be calculated. It is possible 

 also to measure the nuclei and to count the number 

 of nuclei in a fibre. Thus by measuring the diameter 

 and length of the fibre, and then estimating the number 

 and the diameters of the nuclei, the author was able to 

 calculate the relative proportions of the nuclei and the 

 protoplasm. As a matter of fact, the nuclei remain 

 nearly constant in volume, not really quite so, but 

 sufficiently constant to serve as a basis of measurement. 

 Dr. Eycleshymer found that when a Necturus had a 

 length of eight millimetres, it possessed, for each nu- 

 cleus in its muscle fibre, 2737 units of protoplasm, but 

 when it was seventeen millimetres, it possessed for each 

 nucleus, 4318 units per nucleus; at twenty-six milli- 

 metres, 8473 units ; and in the adult, which measures 

 approximately 230 millimetres, it has 22,379 units per 

 nucleus. In other words, as a salamander passes from 

 the eight-millimetre condition, when the development 

 of its muscle fibers is just fairly begun, up to the adult 

 state, when the differentiation of the muscle fibres has 

 been completed, it increases the proportion of pro- 

 toplasmic substance and protoplasmic derivatives from 

 2700 to 22,300 per nucleus. I give round numbers. 

 The increase is approximately eightfold. There is in 

 the adult in the muscle fibre eight times as much 

 protoplasmic substance in proportion to the nucleus 

 as there was at the start of development when the 

 muscle fiber could first be clearly recognised as such. 



