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 geuetic 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 II. 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 
fibrilla; 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 ‘‘Ccelomyaria.” 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 
fibrille 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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