CHAPTER III 

 TWINNING IN EARTHWORMS AND THEIR ALLIES 



One of the earliest studies of one-egg twinning was 

 that of Kleinenberg (1878) on the earthworm Lumbricus 

 trapezoides. The condition described by him is of great 

 interest in our study of twinning, as it was perhaps the 

 first described case of twinning in which it was certain 

 that only one egg was involved. 



Kleinenberg found that of the three to eight eggs 

 found in a cocoon or capsule only one, or occasionally 

 two or three, undergoes development, the others, which 

 he believes to have been unfertilized, undergoing com- 

 plete disintegration within the capsule. Although the 

 author does not lay any stress upon this condition, I 

 would like just here to point out that the environment 

 of the developing egg or eggs in a capsule, fouled by the 

 decay of several other eggs, is not at all likely to result 

 in normal development. 



The embryonic history of the eggs that survive is as 

 follows : Cleavage is apparently normal up to a blastula 

 stage, consisting of a thin-walled, bladder-like vesicle. 

 Gastrulation apparently occurs through the more rapid 

 growth of the cells at one end of the vesicle. At the pole 

 of greater thickness two large cells, the primary mesoblast 

 cells, become pushed in and are overgrown by small 

 surface cells. From these two mesoblast cells there arise 

 by proliferation two rows of cell masses destined normally 

 to be the right and left endodermal and mesodermal 



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