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blasts, are distinguishable from the cells of the peritonaeum by their 

 great size and by the presence of a cell body. They are to be found 

 in every somite with the possible exception of one or more at the an- 

 terior extremity, and represent the «chorda cells« described by Semper 

 in the Naids and Chaetogaster. Very soon after the fission of the worm 

 the neoblasts in the end somite begin to divide and give rise to the 

 greater part of the embryonic tissue that is afterward differentiated 

 into mesodermic structures. This embryonic tissue arranges itself in 

 a very definite way into a median and two lateral elements — the me- 

 dian element apparently consisting of two parts, one from each side. 

 From the median element are derived the ventral mesentery and the 

 walls of the ventral blood vessel. From the lateral elements arise 

 the lateral mesodermic structures with the exception of the circular 

 muscles. 



In very early stages , as soon as the ectoderm and entoderm have 

 extended themselves sufficiently to form a new cavity there are present 

 dorsally, laterally and ventrally small cells that seem to be wholly un- 

 connected with the neoblasts and their products. Of the origin of these 

 cells I can give no positive account. One very clear set of early sec- 

 tions, however, shows with great distinctness cell division taking place 

 in the region of the dorsal peritonaeum just at the posterior limit of 

 the old tissue. After careful examination I am unable to find any 

 connection between this mesoderm and the ectoderm or the ento- 

 derm. I infer, therefore, that the regeneration of the dorsal mesoderm 

 is similar to that of the ventral, but I believe that the tissue in the two 

 regions arises separately. These smaller mesoderm cells give rise to 

 all the circular muscles l and apparently to the dorsal longitudinal 

 muscles and the walls of the dorsal blood vessel. 



The neoblasts are to be regarded as specialized embryonic cells 

 set apart for the rapid formation of new mesodermic tissue immedia- 

 tely upon the fission of the worm. They are present in greater num- 

 ber in the Naids where the formation of new tissue is much more 



1 It appears that the origin of the circular muscles in the regeneration of Lum- 

 briculus differs absolutely from that assigned by Bergh (Zeitschr. f. wiss.Zool. Sep- 

 tember 1890) for the circular muscles in the embryology of Lumbricus. The process 

 of regeneration in Lumbriculus is closely similar to that in Lumbricus and the rege- 

 neration of Lumbricus is apparently a repetition of its embryological development ; 

 hence there are in the regenerating tail of Lumbriculus cell rows that are the equi- 

 valents of those to which Bergh ascribes the origin of the circular muscles, but long 

 before these cell rows are separated from the ectoderm the circular muscles are al- 

 ready distinctly formed and from elements that have no connection with the ecto- 

 derm. 



