REGENERATION IN LUMBRICULUS. 299 



to be derived from the hypodermis, migrating into the coelom to 

 form the cerebral ganglion (p. 295). The fact that many neo- 

 blasts metamorphose and migrate anteriorly is evidence that 

 they probably take part in the building of the new mesoderm in 

 anterior as well as in posterior regeneration. The presence in 

 the bud of other cells of similar appearance makes it practically 

 impossible to follow the laying down of these structures from 

 cells of known origin. It has for some time been agreed by 

 investigators that both endodermal and ectodermal structures 

 are formed in the same manner in both types of regeneration, 

 that is, from the old intestine and body wall, respectively. It 

 seems evident then that the mesodermal structures are produced 

 in the same way at both ends and are not an exception as 

 previously believed. 



An explanation for the formation of the new mesoderm from 

 more or less undifferentiated mesodermal cells rather than from 

 the muscles may perhaps be found in the fact that the cytoplasm 

 of the muscle cells is highly modified. While the cells of the 

 hypodermis and intestine are of a simple, cuboidal or columnar 

 shape, the development of the contractile substance by the 

 muscle cells brings about an extensive modification of the 

 cytoplasm of these cells. Consequently, cells from the peri- 

 toneum, less modified than the others are called upon to form 

 the tissue in the regenerating bud. The development of the 

 new nerve cord is a process not essentially different. In this 

 case the cytoplasm is considerably modified in the formation of 

 fibers and, instead of the nerve cord near the wound dedifferenti- 

 ating to produce the new tissue, the hypodermal cells of the 

 ventral side are called upon to furnish the new material. While 

 these facts may not necessarily indicate that one type of cell is 

 any "more differentiated" than another, the muscle and nerve 

 cells in Lumbriculus certainly are less susceptible to the activating 

 stimulus than are those of the other tissues. 



In regeneration the ectoderm cells enlarge considerably es- 

 pecially those in the median ventral line which may develop 

 nuclei as large as 8 x 1 1 /x with nucleoli between 3 and 4 /* in 

 diameter. Krecker ('10 and '23) is of the opinion that this 

 change in the ventral cells is due to "some influence" of the 



