234 



SOME PROBLEMS OF CELL-ORGANIZATION 



minute central granule or centriole. This discrepancy between Bovcri 

 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 



Fig. 109. bpermatogonium of saiaman- 



below. Above is the 



o f a ring of microsomes. And 



thllS Van Beneden's Oriinal view 



enormous aster, the centrosome at its centre, is Confirmed, that not Only the 



its rays showing indications of nine concentric t h j b t j th centro _ 

 circles of microsomes. The area within the 



second circle probably represents the " attrac- Sphere, IS but a modified area of the 



tion-sphere " of Van Beneden. 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 

 divide 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 



