1910] Johnson: Quantitative Study of Salpa Chain. 1(17 



S. zonaria-cordiformis, on the other hand, has no interme- 

 diate piece. Also in this species there is no intervening un- 

 segmented portion among the young segments of the stolon, and 

 the first block ends abruptly. 



As segmentation proceeds in the same way for all, e.g., by an 

 infolding of the ectoderm, thus cutting the stolon into segments, 

 the differences must arise from differences in rate or continuity 

 of the growth process. Two growth processes occur: viz.. out- 

 ward growth of the stolon, and advance of segmentation. Suppose 

 these two processes to be periodic, exhibiting alternate periods of 

 activity and inactivity. Suppose further that these periods of 

 growth of stolon and progress of segmentation coincide. Would 

 nut the result be the condition we have described above? The 

 blocks would represent periods of activity in growth and seg- 

 mentation. The intermediate pieces would be the result of the 

 period of slower growth, while the absence of such a piece would 

 indicate a complete cessation of growth between the periods of 

 activity. 



This hypothesis would account for the marked difference in 

 the size of the blocks. One period of growth would mark out a 

 block of zooids; a second would arrange the zooids of the block 

 in double file; while a third would bring the zooids to the size of 

 those whose measurements are given. 



The salpa chain then presents an obvious periodicity, at least 

 in its advanced stages of development. Do we not find a parallel 

 to this in plants in what the botanists term the grand period of 

 growth? 3 This phenomenon is defined thus in Pfeffer's Physi- 

 ology of Plants (Pfeffer, TO; vol. II, p. 6) : "Every cell and 

 part of a cell, every organ and the entire plant as well, pass more 

 or less rapidly through a specific progress of development whose 

 character is primarily determined by internal causes. This 

 grand period of growth has definite limits and attains a maxi- 

 mum at a particular phase of development, besides frequently 

 exhibiting secondary maxima. This law applies to animals as 

 well as to plants, and it is not surprising that the grand periods 

 for different organs, and even for special functions also, do not 

 necessarily coincide either in amount or in time. ' ' 



3 This question is raised by Professor Ritter and discussed by him at 

 length in a general book now ready for publication entitled, ' ' Living 

 Things, A Study of the Nature of the Organism.'' 



