Albert C. Eycleshymer 393 



It is important to note that the type of nucleus above described is 

 most frequently found, in the 26 mm. stage, at the upper and lower mar- 

 gins of the myotome. The myoblasts in these localities are less densely 

 fibrillated than elsewhere and are to be considered, as many writers have 

 maintained, the more recently formed myoblasts. 



Necturus, 23 cm. {adult).— In the adult of this length the cephalic 

 muscle segments, and consequently muscle cells, measure 4.0 cm. The 

 area of their transverse section is greatly increased as a comparison of 

 Figs. 12 and 13 will clearly show. The cells are separated by a rela- 

 tively large quantity of connective tissue or endomysium. The muscle 

 cells in transverse sections present somewhat different appearances; in 

 those more frequently observed (Plate I, Fig. 13), the fibrillffi appear 

 evenly distributed and have a considerable sarcoplasm among them; the 

 less common appearance is that seen in the contracted muscle cells, in 

 which the fibrillfe are more closely apposed, giving to the cell a denser 

 appearance. 



The nuclei have again sliifted their position. They are no longer 

 situated at the periphery of the cell, as in the 20 mm. stage, but are, for 

 the most part, evenly scattered throughout the sarcoplasm, as shown in 

 Fig. 13. They have also changed somewhat in form, being longer, 

 wider, and flatter. The most striking difference seen when the nuclei 

 of the early and adult stages are compared is the faint staining of the 

 latter. 



The description of the nuclear structures, as shown in Plate IV, is 

 based upon the study of both isolated nuclei and series of transverse 

 sections. A series of transverse sections of an adult nucleus is represented 

 m Figs. 42-Gl. In structure this nucleus is typical, but in form it is less 

 flattened than usual. The nuclear membrane is visible throughout the 

 greater portion of its extent with small karyosomes occasionally in con- 

 tact with it. In some sections, such as shown in Figs. 50, 51, there 

 appears to be a wide layer of karyosomes lying against the membrane. 

 But it will be shown later that this apparent band is in reality due to an 

 infolding of the membrane. I was at first unable to account for these 

 peculiar appearances, which were never seen in profile views (Fig. -41) ; 

 however, it was later observed that these nuclei are always found in 

 fibres which appear much denser in internal structure. This appear- 

 ance is due to the contraction of the fibrill^, which increase in diameter 

 and become closely apposed. Through the pressure thus brought aljout 

 the nuclei become deeply serrated, and when viewed from the end or 

 m transverse section this infolding of the membrane produces the effect 



