Albert C. Eycleshymer 287 



plate is several layers of cells in thickness, and these cells or myoblasts 

 extend from one end of the muscle-plate to the other, as can readily be 

 demonstrated by teasing them apart. The peripheral portion or cutis- 

 plate (Fig. 1, c. j).y consists of a single layer of cells with their long 

 axes radiating from the center of the myotome. The nuclei of these cells 

 even at this early stage are distinctly different from those of the mnscle- 

 plate, not only in form and size, but also in staining capacity. 



The yolk granules in the earlier stages were of uniform size and evenly 

 distributed, indicating an even rate of absorption. At this time they are 

 variable in size, the smaller being at the ends of the myoblast, where they 

 grade off imperceptibly until they are no longer visible with the most 

 efficient lenses. 



This unequal rate of yolk absorption gives rise, as depicted in Figs. 

 1 and 3, to clearer zones at either end of the myoblast. A close study 

 of these clearer ends, after osmic acid fixation, reveals the presence of 

 a more or less distinct longitudinal fibrillation (Fig. 1). These cyto- 

 plasmic striations are more obvious on the notochordal side of the myo- 

 blast, and in many cases converge towards that side at the level of the 

 inner margin of the clear zone. Whether these striae are homogeneous 

 or finely granular cannot be determined, since their structural analysis 

 is beyond the definition of the best optical apparatus. In position and 

 arrangement they correspond so closely to the fibrillae, which are plainly 

 defined in the stages immediately following, that one does not hesitate 

 to assume that they are undifferentiated fibrillae. Little is known of 

 their origin. They may differentiate in situ, or they may represent lines 

 of granules which, earlier scattered, have now become arranged in linear 

 series. If the fibrillge are first formed at the free ends of the myoblasts 

 instead of along the side, as is usually held, the process of fibrillation 

 would be quite in accord with the generally accepted theories of cellular 

 differentiation (cf. E. B. Wilson, 96, 40). 



The nuclei of the myoblasts lie at different levels as shown in Fig. 1. 

 In form they vary from the oval to the obtusely oval represented in Plate 

 II, Fig. 14. Their average length as determined by many measure- 

 ments is about 33 /x and their average widtli 10 /x. There are no indica- 

 tions of a paired arrangement; indeed they rarely lie opposite. Numer- 

 ous mitotic figures are seen, but their spindles are always parallel with 

 the long axis of the myoblast. It is thus oljvious that neither in the posi- 



' I have used the term cutis plate for convenience in designating the outer 

 layer of the primitive myotome, but I do not wish to imply or express an 

 opinion as to the fate of the cells of this layer. 



