20 



WILSON. 



[Vol. II. 



Verrill. These larvae were reared from artificially fertilized 

 eggs, and a full account of their development is in preparation 

 for a subsequent paper. The eggs of this nemertean are 

 opaque during cleavage and gastrulation, but become beautifully 

 transparent on reaching the pilidium stage. 



The mesenchyme first appears as isolated cells derived from 

 the ectoderm, as observed by Metschnikoff {Zeit. f. wiss. ZooL, 

 Bd. xxxix). 



They move about freely in the gelatinous liquid which fills 

 the space between ectoderm and entoderm. At first they are 

 nearly spherical in outline, but they soon begin to develop 

 processes and become branched, in which condition they are 

 very readily distinguished from the other elements. So long 

 as they remain free floating there is no indication of cell fibers, 

 but simply a nucleus enclosed in cytoplasm. But as they begin 

 to branch they grow larger, and granules appear in the cyto- 

 plasm, while the branches become gradually fibrous in structure. 

 No amitotic division stages, however, were noticed in any of 

 these cells, such as were found by Montgomery in the free- 

 floating mesenchyme cells of the adult worm {Zool. Jakrb., 

 Bd. x). The branches hinder the freedom of motion of the 

 cells, and the latter gradually become fixed in position. 



The fibrils at the extremities of the branches are then 

 fastened in place, and from being mere wandering mesenchyme 

 the cell becomes one of the muscles of the pilidium. This 

 transformation was watched several times in the formation of 

 different muscles, and nearly all the intermediate stages were 

 observed. The most important muscle of the pilidium is the 

 one which extends from the apical plate downward to the 

 anterior border of the lappets. The development of this muscle 

 was watched in many different individuals. When the mesen- 

 chyme cells first develop branches, one of them can be seen to 

 become stationary in about the position of the future apical 

 muscle. One of its processes becomes fastened to the apical 

 plate, while another fastens to the wall of the digestive tract, 

 and sometimes a third connects with the aboral wall of the 

 pilidium (Fig. 4). The number of processes is not constant, 

 but the position assumed by the cell is approximately so. 



