DEDIFFERENTIATION IN ECHINUS LARWE. 213 



out of the tissue and adherent to it externally (these cells appeared 

 normal, in contradistinction to the pathological granular disinte- 

 gration of the tissue of the arms). The form of the gut was 

 well preserved, and no disintegration of any sort was visible in 

 it (cf. Section 2, 5). 



In some cases it appeared that all tissues had disintegrated with 

 the exception of the somewhat contracted gut, which was thus 

 left by itself ; but I was unable to be quite sure of this. 



2. Slightly shrunken plutei with the skeleton protruding through 

 the arms; these latter might be from 25 to. 75 per cent, of their 

 original length. 



3. As (2) but the skeleton not protruding from the arms. The 

 terminal portions of the spicules had apparently been resorbed 

 by the arms as they contracted. 



All the plutei had sunk to the bottom, and ciliary action was so 

 much reduced that only very sluggish movement was taking 

 place. 



In addition to the types mentioned, others were seen which 

 were of spheroidal form, dense, without arms, and with simple 

 or no spicules. I at first thought that these might be examples 

 of extreme dedifferentiation, but on examining the control culture, 

 I found at the bottom a certain proportion of quite similar organ- 

 isms. The culture was slightly overcrowded and these were 

 doubtless susceptible individuals inhibited by the slightly adverse 

 conditions. These types are mentioned to draw attention to the 

 necessity for careful control in similar experiments. (This has 

 already been emphasized by Shearer, De Morgan and Fuchs ('13), 

 p. 274, etc.) They are also interesting as indicating that con- 

 siderable inherited variations in vigor and resistance occur among 

 the offspring of a single pair of Echinus. 



In the later experiments now to be recorded, the plutei were 

 picked out from the control culture with a fine pipette under the 

 binocular microscope, transferred to the medium to be tested and 

 there examined, to see if any of these minute armless forms had 

 been transferred by mistake. In no experimental vessel was more 

 than one of these forms discovered immediately after transfer; 

 it can therefore be safely assumed that in this series the experi- 



