34 GENERAL SKETCH OF THE CELL 



nar epithelium, the threads in the peripheral part of the cell often 

 assume a more or less parallel course, passing outwards from the 

 central region, and giving the outer zone of the cell a striated appear- 

 ance. This is very conspicuously shown in ciliated epithelium, the 

 fibrillse corresponding in number with the cilia as if continuous with 

 their bases (Fig. 13).^ In nerve-fibres the threads form closely set 

 parallel fibrillae which may be traced into the body of the nerve-cell ; 

 here, according to most authors, they break up into a network in 

 which are suspended numerous deeply staining masses, the "chromo- 

 philic granules" of Nissl (Fig. 15). In the contractile tissues the 

 threads are in most cases very conspicuous and have a parallel course. 

 This is clearly shown in smooth muscle-fibres and also, as Ballowitz 

 has shown, in the tails of spermatozoa. This arrangement is most 

 striking in striped muscle-fibres where the fibrillae are extremely well 

 marked. According to Retzius, Carnoy, Van Gehuchten, and others, 

 the meshes have here a rectangular form, the principal fibrillae having 

 a longitudinal course and being connected at regular intervals by 

 transverse threads ; but the structure of the muscle-fibre is probably 

 far more complicated than this account would lead one to suppose, 

 and opinion is still divided as to whether the contractile substance 

 is represented by the reticulum proper or by the ground-substance. 



Nowhere, however, is the thread-work shown with such beauty 

 as in dividing-cells, where (Figs. 16, 24) the fibrillae group themselves 

 in two radiating systems or asters, which are in some manner the 

 immediate agents of cell-division. Similar radiating systems of fibres 

 occur in amoeboid cells, such as leucocytes (Fig. 35) and pigment- 

 cells (Fig. 36), where they probably form a contractile system by 

 means of which the movements of the cell are performed. 



The views of Biitschli and his followers, which have been touched 

 on at p. 18, differ considerably from the foregoing, the fibrillae 

 being regarded as the optical sections of thin plates or lamellae 

 which form the walls of closed chambers filled by a more liquid 

 substance. Biitschli, followed by Reinke, Eismond, Erlanger, and 

 others, interprets in the same sense the astral systems of dividing- 

 cells which are regarded as a radial configuration of the lamellae 

 about a central point (Fig. 8, B). Strong evidence against this view 

 is, I believe, afforded by the appearance of the spindle and asters 

 in cross-section. In the early stages of the egg of Nereis, for 

 example, the astral rays are coarse anastomosing fibres that stain 

 intensely and are therefore very favourable for observation (Fig. 43). 

 That they are actual fibres is, I think, proved by sagittal sections 

 of the asters in which the rays are cut at various angles. The 



^ The structure of the ciliated cell, as described by Engelmann, may be beautifully 

 demonstrated in the funnel-cells of the nephridia and sperm-ducts of the earthworm. 



