MUSEUM OF COMPARATIVE ZOOLOGY. 149 



made up of the ends of muscle cells and to what extent it is formed of 

 other elements. Certain it is that this zone contributes to the forma- 

 tion of at least a certain number of other elements : this may be by a 

 differentiation from the protoplasmic body of the muscle cell, or it may 

 be that the elements have no genetic connection with the muscle cells. 

 In order to discuss this question it will be necessary first to consider 

 carefully the structure of the individual muscle cell. 



For this study the region near the dorsal or ventral line is very favor- 

 able, since here the cells are shorter and broader than elsewhere, and 

 thus it is easier to trace the cell walls. Cross sections of this region 

 (Plate IT. Fig. 21) show with perfect clearness that each muscle cell is 

 composed of two portions, corresponding in appearance and position to 

 the two zones of the muscular layer. The highly refractive peripheral 

 portion is seen in longitudinal section (Fig. 22) to consist chiefly of 

 fibrillaD ; these are, however, developed only at the periphery of this 

 portion of the cell, the core of which is composed of a finely granular 

 protoplasm. The latter is directly continuous with the granular proto- 

 plasm of which the deep-seated part of the cell is exclusively composed 

 and in which the nucleus is located. This is the condition of the typi- 

 cal muscle cell of the " Coelomyaria." In the centre of the muscu- 

 lar layer, i. e. in the lateral walls of the body, the cells diffe. only in 

 being much deeper and more flattened. From maceration preparations 

 (Fig. 22) it may be seen that the inner or deep margin of the band of 

 fibrillse is bounded by a very thin layer of protoplasm which at inter- 

 vals is continued downward into the elongated cell body. This is also 

 seen in transverse sections (Figs. 28-30). It would seem (Fig. 22) as if 

 each muscle cell had more than one protoplasmic prolongation, but since 

 I was unable to ascertain the length of the individual cells this cannot 

 be positively asserted. 



The nuclei usually lie just below the contractile portion of the cell. 

 They are oval, and each has a thick nuclear membrane, which stains 

 deeply, but encloses very little stainable substance, the numerous nucle- 

 oli being minute and faint. There are also at times unquestionably as 

 many as two nuclei in each protoplasmic projection, and in certain speci- 

 mens it was common to find nuclei far down toward the deep end of 

 the cell. 



I must call attention in this connection to some very peculiar nuclei 

 which were found among the nuclei of the muscle cells, but which differ 

 from them strikingly (Fig. 27). They were usually located rather more 

 distally than the others, and each showed a tail of varying length, which 



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