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E. M. RUN VAN AND H. B. TORREY. 



last phases of the process of fission, have appeared in a circlet 

 near the aboral pole (Fig. i). 1 



Why do such cilia not appear on the stalked zooid? 



It may be noticed in the last stages of fission that the zooid 

 destined to become free retains its connection by a slender proto- 

 plasmic strand with the body of the stalked zooid, not directly 

 with the stalk itself. This fact suggests what has proven to be 

 the correct view, namely, that cilia which would normally develop 

 on every individual are able to show themselves only when 

 sufficiently isolated physiologically from the stalk. Such isola- 

 tion exists .when the connection between the separating zooids 

 is reduced to a narrow strand. 



8 



This view was reinforced by the familiar fact that, upon 

 becoming attached to the substrate, the free zooid gradually 

 loses its cilia as its stalk develops. In the normal life history, 

 then, aboral cilia develop in isolation from the stalk and disappear 

 with the development of the stalk. 



The test was applied by cutting a stalked zooid quite away 

 from the stalk. This was accomplished under a binocular, by 



1 The figures have not been drawn with a camera. Their scale varies somewhat. 



