288 Changes in the Muscle Cell of Xecturus 



tion of the nuclei nor in their plane of division does one find evidence 

 of a longitudinal splitting of the myoblast. 



The following description of the nuclear structures * is based upon 

 the study of the nuclei both in teased preparations and in series of trans- 

 verse sections. 



The nuclear membrane (n. m.) is here and there obscured by a layer 

 of karyosomes (Plate II, Figs. 21, 22, 25, 26), but is in general well 

 defined. In places the cytoplasmic reticulum appears as threads ter- 

 minating in the membrane. 



The linin network (/) is of fairly uniform character in all the prepa- 

 rations. The large open spaces seen in some of the sections (Figs. 18, 

 21, etc.) are artifacts due to imperfect cutting. The threads of this 

 delicate network pass into the nuclear membrane, but I have l)een unable 

 to find any indications of a continuity between them and the cytoplasmic 

 reticulum; the latter is not only poorly defined, but also less deeply 

 stained by nuclear dyes, leading one to regard the linin network and 

 cytoplasmic reticulum as fundamentally different. 



Three or four plasmosomes (/;/.) are usually present in each nucleus. 

 They are rarely found at the periphery, but otherwise show no constancy 

 in position ; they are usually spherical, fairly uniform in size, surrounded 

 by a sheath of deeply staining granules, but themselves possess only 

 slight affinity for nuclear stains. 



The karyosomes (I'l/.) shown in Plate II are numerous and variously 

 distributed in the different nuclei. In many there is a peripheral zone 

 which is comparatively free from these structures. In some there is no 

 particular arrangement with reference either to periphery or axis. They 

 are usually irregular in form and size, with numerous processes which ex- 

 tend along the linin threads and form anastomoses with adjoining karyo- 

 somes. 



Necturus 7-8 mm,. — As development proceeds, the clear zones at eitlier 

 end of the myoblast become wider, owing to the continued absorption of 

 the yolk granules, and one is thereby enabled to make out more definitely 

 the relation of the striated or fil)rillated tract to the other portions of the 

 cell. The fibrillse have extended farther along the notochordal side of 

 the cell and appear as represented in Fig. 4, Plate I, being grouped in 

 brush-like form with the point of convergence (a) at the level of the 

 inner margin of the clear zone (c. z.). I have repeatedly looked for 

 eentrosomes at the points of convergence, but without success. The 



* In describing the changes in nuclear structures I have adopted the ter- 

 minology used by E. B. Wilson. g6. 



