REGENERATION IN LUMBRICULUS. 287 



By the third day these, too, are enlarging so that it is not possible 

 to discover a line of demarcation between the gut of the bud and 

 that of the old part. At about this time double nucleoli and 

 mitotic figures begin to appear in the new gut. Cell proliferation 

 in the new tissue thus increases as that in the old part is de- 

 creasing. Consequently, as far as the intestine is concerned, 

 regeneration usually ceases between the sixth and seventh days. 

 After this time it is a matter of growth in the newly formed 

 tissue itself. 



Mesoderm. 



The method of formation of the new mesoderm is not as 

 generally agreed upon as is that of the new intestine. Hepke 

 ('97) in Naids, von Wagner ('06) in Lumbriculus and Abel ('02) 

 in Tubifex and Nais, among others, maintain that the new 

 mesodermal structures originate from ectodermal cells which 

 migrate into the ccelom. Randolph ('92) and Iwanow ('03) in 

 different species of Lumbriculus and Krecker ('10 and '23) in 

 Tubifex and Limnodrilus find that these structures are derived 

 from relatively unmodified cells of the mesoderm to which the 

 term neoblasts was first applied by Randolph. Krecker ('23) 

 reports that they are ordinarily found on the posterior surfaces 

 of the septa in a quiescent state. After a worm is cut they 

 enlarge to 8 or 10 times their former size and then migrate along 

 the nerve cord to the wound region. According to his data 

 these cells are activated on the seven septa nearest the wound 

 w r ith those in the four nearest segments giving the greatest 

 response. In my preparations of individuals which have been 

 regenerating for either three or four days a number of clearly 

 distinguishable neoblasts are always present as far as eight or 

 nine segments from the wound. Two examples, one of a three-, 

 the other of a four-day regenerate, are given in Table IX. All 

 the cells which lie upon the septa and are clearly neoblasts with 

 nucleoli of 2 /z or over are included in these counts. All are found 

 on the posterior surfaces of the septa and are confined almost 

 entirely to the ventral portion. These cells correspond pre- 

 sumably to the later stages of metamorphosis described by 

 Krecker ('23). In both of these cases nine segments are clearly 

 contributing neoblasts while the tenth and eleventh are ap- 



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