Johnson: Quantitative Study of Salpa chain. 



I.v; 



facing the same way. Now let them face aboul so that the backs 

 of those in one row are turned towards the backs of those in the 

 other row. They will now illustrate two rows of salpae." 



"To make the illustration more perfect, suppose that, instead 

 of stepping into their new places, the soldiers grow until they 

 are pushed out by mutual pressure, and suppose thai their Inn, Is. 

 growing fastest, form two rows while their feel still form one row, 

 and suppose furthermore that as inch soldu r rotaU s, his f, < I I urn 

 first and that the twist runs slowly up his body to his head which 

 turns last. 2 We must also imagine that these various changes all 

 go on together, and thai while they are taking place, each soldier 

 not only grows larger, but also developes from a simple germ 

 to his complete structure." _ 



This is an admirable description of the essential facts of 

 segmentation and shifting of position. The region which I have 

 called the "deploying point," which is shown in fig. 4, p' , is the 

 point where the zooids change from single to double file. 



A study of the longitudinal sections of a chain shows that the 



segmentation is. as in 8. pinnata, initiated by the ectoderm. 



Folds, or undulations, appear in the ectodermal layer at regular 



intervals along the length of the chain. (Figs. 7. S. and 12.) 



The raised part of these folds 



-*§iii becomes thicker while the con- 



S g strictions marking the divisions 



■^i? «fk between segments are but little, 



''■''{<*J$ if any. thinner than the original 



" -; ectoderm. The constrictions in- 



■'■'Skl^ crease in depth, cmtting into the 



other structures and dividing 



%a!sJP them up into segments. The 



,-:, inner strands follow the lead of 



ityjagy * ne ectoderm in segmenting, the 



undulations in them appearing 



; ; first where their surface is in 



n e contact with the ectoderm. No 



Fig. 7.— Longitudinal section trace of the division appears 



through the first block of a chain a i ong tn ese inner strands until 



of S. fusi form/* rundnata. , . , ^ 



the ectoderm reaches them. 



9 



- The italics in the above citation are mine. — M. E. J. 



