304 SOME PROBLEMS OF CELL-ORGANIZATION 



microsomes arranged in a network. The clearest evidence is, however, 

 afforded by the origin of the spindle-fibres in mitotic division ; for it 

 is now well established that these may be formed either inside or out- 

 side the nucleus, and at the close of mitosis the central portion of the 

 spindle appears always to give rise to a portion of the cytoplasm 

 lying between the daughter-nuclei. In such a case as that of the 

 sea-urchin (see above) we have, therefore, evidence of a direct trans- 

 formation of chromatin into linin-substance, of the latter into spindle- 

 fibres, and, finally, of these into cytoplasm. 



When all these facts are placed in connection, we find it difficult to 

 escape the conclusion that no definite line can be drawn between the 

 cytoplasmic granules at one extreme and the chromatin-granules at 

 the other. And inasmuch as the latter are certainly capable of 

 growth and division, we cannot deny the possibility that the former 

 may themselves have, or arise from elements having like powers. 

 But while we may take this as a fair working hypothesis, we should 

 clearly recognize that the base of well-determined fact on which it 

 rests is approached by a circuitous route ; that in case of most of the 

 cytoplasmic granules there is not the slightest evidence that they 

 multiply by division ; and that even though some of them may have 

 such powers, we cannot regard them as the ultimate structural units, 

 for the latter must be bodies far more minute. 



E. THE CENTROSOME 



From our present point of view the centrosome possesses a peculiar 

 interest as a cell-organ which may be scarcely larger than a cytomi- 

 crosome, yet possesses specific physiological properties, assimilates, 

 grows, divides, and may persist from cell to cell without loss of identity. 

 Nearly all observers of the centrosome have found it lying in the 

 cytoplasm, outside the nucleus ; but apart from the Protozoa (p. 94) 

 there is at least one well-established case in which it lies within the 

 nucleus, namely, that of Ascaris, where Brauer made the interesting 

 discovery that in one variety (imivalens) the centrosome lies inside the 

 nucleus, in t/ie other variety (bivalens} outside a fact which proves 

 that its position is non-essential (cf. Figs. 120 and 148). 



An intra-nuclear origin of the centrosome has also been asserted by 

 Julin ('93) in the primary spermatocytes of Styleopsis, by Riickert 

 ('94) in the eggs of Cyclops, Mathews ('95) in those of Asterias, Car- 

 noy and Le Brun ('97, 2) in Ascaris, Van der Stricht ('98) in the eggs 

 of Thysanozoon, by R. Hertwig ('98) in Actinosphcerium, Calkins 

 ('98, i) in Noctiluca, and Schaudinn ('96, 3) in spore-producing buds of 

 Acanthocystis, though in the last-named form the centrosome of the 

 vegetative forms is extra-nuclear (p. 92). 



