234 



SOME PROBLEMS OF CELL-ORGAXIZATION 



minute central granule or ccntriolc. This discrepancy between Boveri 

 and Van Beneden was cleared up in a measure by Heidenhain's 

 beautiful studies on the asters in leucocytes, and the still more 

 thorough later work of Driiner on the spermatocyte-divisions of the 

 salamander. In leucocytes (Fig. 35) the large persistent aster has at its 

 centre a well-marked radial sphere bounded by a circle of microsomes, 

 as described by Van Beneden, but without division into cortical and 

 medullary zones. The astral rays, however, show indications of other 



circles of microsomes lying out- 

 side the centrosphere. Driiner 

 found that a whole series of such 

 concentric circles might exist (in 

 the cell shown in Fig. 109 no 

 less than nine), but that the inner- 

 most two are often especially 

 distinct, so as to mark off a cen- 

 trosphere composed of a medul- 

 lary and a cortical zone precisely 

 as described by Van Beneden. 

 These observations show conclu- 

 sively that the centrosphere of 

 the radial type is merely the inner- 

 most portion of the aster, which 

 acquires an apparent boundary 

 through the especial development 

 of a ring of microsomes. And 

 thus Van Beneden's original view 

 is confirmed, that not only the 

 aster as a whole, but also the centro- 

 sphere, is but a modified area of the 

 general cytoplasmic thread-work. 

 Heidenhain points out that there are many cases — for instance, 

 the young sperm-aster — in which there is at first no clearly marked 

 central sphere, and the rays proceed outward directly from the centro- 

 some. The sphere, in such cases, seems to arise secondarily through 

 a modification of the inner ends of the astral rays. Heidenhain there- 

 fore concludes that the centrosome is the only constant element in the 

 sphere, the latter being a secondary formation and not entitled to rank 

 as a persistent cell-organ, though it may in certain cases persist and 

 dividt^ like the centrosome. Vom Rath, who has made a very careful 

 study of the attraction-spheres in a large number of cells among both 

 vertebrata and invertebrata, arrives at a nearly similar view, though 

 he lays greater stress on the differentiation and independence of the 

 sphere. In asters of dividing cells he could find in many cases no 



Fig. 10^. — SnjcniKitogonium of salaman- 

 der. [DiU'NF.R.] 



The nucleus lies below. Above is the 

 enormous aster, the centrosome at its centre, 

 its rays showing indications of nine concentric 

 circles of microsomes. The area within the 

 second circle probably represents the " attrac- 

 tion-sphere " of Van Beneden. 



