CERTAIN GENERAL CHARACTERISTICS 



27 



shows four stages in development of a chain in the rhabdocoel, Steno- 

 stomuvi. The second and following fissions occur in order from anterior to 

 posterior zooid of the chain; that is, increase in length of a zooid sufficient 

 to permit determination of a new head region, which can occur only at a 

 certain distance from the head of the zooid, is most rapid in the anterior 



A 



nil 



^iii 



V-^II2 



Fig. 10. — Four stages in development of zooid chain of Stowstomum grande; fission zones of 

 first, second, and third order are indicated by Roman numerals; the order of their appearance 

 in second and third series, by Arabic numerals. 



zooid and less rapid from zooid to zooid posteriorly, as the figure indicates. 

 Consequently, the chain shows a gradient pattern as regards order of ap- 

 pearance of new zooids, and each fission plane apparently results from de- 

 termination of a new head region in consequence of a certain degree of 

 physiological isolation of the body-level concerned from the dominance of 

 the head region of that zooid. Among microdrilous oligochetes similar pat- 



