294 LEONARD P. SAYLES. 



Mesoderm. 



Many investigators Hepke ('97), von Wagner ('oo) and Abel 

 ('02) among others are of the opinion that the mesoderm both 

 of anterior and of posterior regenerates is formed from the 

 ectoderm. The weight of evidence in posterior regeneration, as 

 previously mentioned, now seems to favor mesoderm formation 

 from neoblasts. In anterior regeneration Iwanow ('03) in Lum- 

 briculus variegatus and Krecker ('10) in Tubifex and Limnodrilus 

 find that the new mesoderm is formed from the old mesoderm 

 and not from regeneration cells as in posterior regeneration. 

 They find a distinct fraying out of the longitudinal muscles at the 

 wound. Many of the cells then lose their contractile substance 

 and wander about in the coelom where they become mixed with 

 other cells peritoneal, connective tissue and ectodermal so 

 that it is "hard to distinguish between the various types" 

 (Krecker, '10, p. 441). After about the fifth day of regeneration 

 they begin to develop contractile substance and to become 

 arranged in muscle masses. The production of the body muscu- 

 lature is much slower in anterior than in posterior regeneration. 

 "In specimens killed three weeks after the operation the muscu- 

 lature of this region was still in a very undeveloped state" 

 (Krecker, '10, p. 442). 



As the matter stands at present, then, investigators are divided 

 into two groups regarding the formation of the new mesoderm. 

 Those in one believe that it is derived from the ectoderm, those 

 in the other that it is produced by a partial dedifferentiation 

 followed by a redifferentiation of the cells of the old mesodermal 

 structures. Those who support an ectodermal origin of the 

 mesodermal structures believe that the mesoderm is formed in 

 the same manner at both ends. Those of the other group, 

 however, think that the origin is due to one type of mesodermal 

 cell at the posterior end the neoblasts and to another at the 

 anterior end the specialized cells which dedifferentiate. 



In anterior regeneration, just as in posterior regeneration, there 

 are many opportunities for confusion. In the former it is even 

 more difficult than in the latter to follow just what happens. 

 The migration of ectoderm cells into the coelom in the formation 

 of the cerebral ganglion might easily mislead one. In addition 



