REGENERATION IN LUMBRICULUS. 303 



maximum size beyond which it cannot exist as an individual 

 droplet under the conditions existing in the nucleus. This view 

 is in accord with our knowledge of the limitations to drop size 

 in emulsions. 



After one day of regeneration many cells show this increase in 

 the amount of nucleolar material, as evidenced by the occurrence 

 of a number of double nucleoli as well as the enlargement of the 

 single ones. On the second and third days the frequency of 

 double nucleoli reaches a maximum and after that time falls 

 off slowly. Following the increase in nucleolar material, there 

 appear numerous cases of mitosis. It seems probable that this 

 increase in nucleolar substance is indicative of a heightened 

 activity on the part of the cells in preparation for cell division. 



A comparison of the amount of nucleolar material in the 

 various types of cells taking part in regeneration reveals a distinct 

 parallelism between this amount and the relative activities of 

 these cells. There are four general types of cells involved: 

 (i) those of the old gut which form the new gut; (2) those of 

 the old dorsal and lateral hypodermis which build the new 

 hypodermis; (3) those of the old ventral hypodermis of which 

 the special function is to furnish the material for the nerve cord 

 in the regenerating bud ; finally (4) the neoblasts which form the 

 new mesodermal structures. Of these, the cells of the first two 

 types maintain to a certain extent their usual epithelial arrange- 

 ment, only a comparatively small amount of migratory activity 

 occurring. Their nucleoli enlarge considerably but by no means 

 as much as in the case of the other two types. The ventral 

 ectoderm cells and the neoblasts, when fully transformed, are of 

 about the same general size and appearance and have nucleoli of 

 nearly twice the diameter of those in the other cells. The ventral 

 ectoderm cells must naturally undergo rapid proliferation to 

 supply all of the material necessary for the nerve cord; the 

 neoblasts must migrate to the wound region and there multiply 

 with considerable rapidity. There is some relation in Lum- 

 briculus, then, between the functional activity of the cells and 

 the amount of nucleolar material present in them. This is a 

 conclusion similar to that drawn by Ludford ('22) from a study 

 of Limnxa with particular reference to the behavior of the 



nucleolus in oogenesis and cleavage. 

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