476 MONTGOMERY. [Vol. XV. 



tially similar to those of Lineus ; in the metanemerteans they 

 are too small for satisfactory study.) 



These nuclei are very variable in shape, all extremes being 

 found between ovoid or oval and elongate rod-like forms. But 

 they are rarely angular. I have remarked in a previous con- 

 tribution that the nuclei of the muscle cells are more variable 

 in form than those of the cells of any other tissue in the 

 nemerteans, and now I would offer the following explanation 

 for this variability : when the muscle fiber (a single, smooth 

 fiber with attached nucleus constitutes a muscle cell) contracts, 

 this contraction must produce likewise a contraction (shorten- 

 ing) of the nucleus ; but when the fiber expands the form 

 of the nucleus must become more elongate, corresponding to 

 the elastic extension of the fiber, for the fiber cannot contract 

 without causing a shortening of its nucleus, since the latter is 

 closely adherent to it. 



One very small nucleolus is usually to be seen in each 

 nucleus (Figs. 51-54, 56); sometimes it does not appear to be 

 present (Fig. 55), but whether in these cases it is absent or only 

 escapes observation by reason of its minute size, it is difficult 

 to decide ; in the greater number of nuclei it may be seen by 

 careful focussing of the microscope. It most usually lies very 

 close to the center of the mass of nucleoplasm, so that if the 

 nucleus be larger at one pole than at the other it is situated in 

 the larger end, while in elongate nuclei, of nearly equal diam- 

 eter throughout, it usually lies at an early equal distance from 

 both ends of the nucleus. The nucleolus may be said, as a 

 general rule, to occupy the center of the nuclear substance, 

 and is not often markedly excentric ; in none of the other cells 

 examined in the course of these investigations did the nucleoli 

 show a similar tendency to occupy the center of the nucleus. 

 The nucleolus always stains differently from the chromatin. 



The relative amount of chromatin varies in different nuclei. 

 It is always found, after the action of various fixatives, to occur 

 in the form of small granules, which are connected by delicate 

 irregular fibers, which stain exactly as the granules do. The 

 nuclear sap stains faintly with haematoxylin (this has not 

 been shown in the figures). The nucleolus is either in contact 



