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DR ARTHUR T. MASTERMAN ON THE 



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6.— Diagram illustrating the Segmentation of the Ccelom in Echinodermata. 

 l.l' = left and right elements of pre-oral ccelom (1. only is pre-oral ccelom). 

 2. 2' = left and right lateral ccelom (hydroccele and aboral ccelom respectively). 

 3. 3' = left and right posterior ccelom (3. only is posterior ccelom). 

 The small round vesicle in 1, 3, and 6 is the central ccelom. 



1. As seen in development of Cribrella ; the perfect bilaterality is characteristic. 



2. As seen in Ophiura brevispina, according to Grave. There is some dou'ot about 1' and 2', but the presence of 

 anterior and posterior ccelomic rudiments appears certain. 



3. As seen in Echinus esculentus, according to MacBride. This should be compared with diagram 5. The main fact 

 of importance is the early formation of right and left, rather than anterior and posterior elements. Asymmetrical origin 

 of the true posterior ccelem (3) seems indicated. 



4. As seen in Synapta digitata, according to Semon (Clark). The asymmetry is even more pronounced. 2' and 3 

 are at one time together. 





