ORIGIN OF THE MESODERM AND BEHAVIOR OF 



THE NUCLEOLUS IN REGENERATION 



IN LUMBRICULUS. ^ 



LEONARD P. SAYLES. 



INTRODUCTION. 



The microdrilous annelids have been the center of considerable 

 interest on the part of students of regeneration particularly from 

 the point of view of the origin of the cells which form the new 

 tissue. While the production of new endodermal and ectodermal 

 structures from cells of the corresponding old parts is generally 

 accepted, the origin of the mesoderm in the bud has been a point 

 of contention. Hepke ('97) in Naids, von Wagner ('oo and '06) 

 in Lumbriculus and Abel ('02) in Tubifex and Nais, among 

 others, are of the opinion that the new mesodermal structures 

 both in anterior and in posterior regeneration are of ectodermal 

 origin. Randolph ('92) in posterior regeneration in Lumbriculus 

 reports their formation from comparatively unmodified cells of 

 the old mesoderm to which she gives the name "neoblasts." 

 Iwanow ('03) in Lumbriculus and Krecker ('10) in Tubifex and 

 Limnodrilus verify her conclusions. They find that in anterior 

 regeneration, however, cells from old specialized mesodermal 

 structures form the new portion. One of the features in the 

 descriptions of the cells of the bud region by these workers is 

 the prevalence of large nucleoli in the cells involved in the 

 regenerative activity. Krecker ('23) gives a rather complete 

 description of the origin and migration of the neoblasts in 

 posterior regeneration. Studies on the origin of the new meso- 

 derm in regeneration together with observations on nucleolar 

 changes in these and other tissues are reported in this paper. 

 For invaluable advice on this work I am indebted to Dr. J. W. 

 Wilson of Brown University at whose suggestion the problem 

 was first undertaken. 



1 This paper forms a part of a thesis presented in partial fulfilment of the require- 

 ments for the degree of Doctor of Philosophy in Brown University, June, 1926. 



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