176 CHAS. B. WILSON. 



unicellular, consisting of a cell body with a distinct nucleus, 

 and with muscle-fibres extending from it in two directions. 

 Here^ then, the branches become fibrous as well as the body 

 of the cell. 



When the streaming movement necessary to the sending 

 out of spin-threads is stopped by contact with something, the 

 threads at once become striated. At first the striae are 

 hardly visible and some distance apart, but they rapidly 

 increase in size and distinctness until finally they occupy 

 nearly all the muscle strand. It seems probable that this 

 increase in fibrillation consists largely of an assimilation of 

 the non-fibrillated portion. During this transformation the 

 whole cell body is often conveyed out into the incipient 

 muscle sti'and, where it is converted into muscle fibrillte. 



3. Fibrillation appears to start in a rearrange- 

 ment of the cytoplasmic reticulum, whereby its 

 threads, instead of forming an irregular network, 

 become parallel. 



We can see how a streaming of the cytoplasm along the 

 spin-thread pseudopodium would help such an arrangement, 

 for the fibrillfe are longitudinal. Several other facts support 

 such an origin of muscle fibrillation. The familiar radiating 

 fibrillsB in the asters of dividing cells, when followed out- 

 ward, can be traced into the cytoplasmic reticulum, and 

 they are so continuous with its threads that it is impossible 

 to tell where the fibril ends and the reticulum begins, or vice 

 versa. 



Again, the muscle fibrillae under favourable conditions can 

 be followed into the cell to which they are attached, and 

 terminate there in the cytoplasmic network, with which they 

 are so completely fused, that once more it is impossible to 

 tell where one ends and the other begins. Furthermore, 

 in a study of columnar epithelium it may often be noticed 

 that the cytoplasm in the outer zone of the cell has been 

 arranged in more or less parallel fibrillae along the long 

 diameter of the cell. No reticulum is visible in this part of 

 the cell except that formed by the fibrillae. In figures of 



