THE CEXTROSOME 305 



As already stated, ^ it is still undetermined whether a true centro- 

 some may ever arise de novo, but the evidence in favour of such a 

 possibility has of late rapidly increased. Carnoy ('86) long since 

 showed that the ^g^ of Ascaris, during the formation of the polar 

 bodies, sometimes showed numerous accessory asters scattered 

 through the cytoplasm. Reinke ('94) described somewhat similar 

 asters in peritoneal cells of the salamander, distinguishing among 

 them three orders of magnitude, the largest containing distinct 

 centrosomes or "primary centres," while the smaller contained 

 "secondary" and "tertiary" centres, the last named being single 



H 





D 



E 



a: 







F ^ 



Fig. 148. — Mitosis witli intra-nuclear centrosome, in the spermatocytes of Ast\iris tttfx<^h- 

 cephala, var. univalens. [Brauer.] 



A. Nucleus containing a quadruple group or tetrad of chromosomes (/). nucleolus («). and 

 centrosome {c). B. C. Division of the centrosome, D. E.F. G. Formation of the mitotic tiijure. 

 centrosomes escaping from the nucleus in G. 



microsomes at the nodes of the cytoreticulum. By successive aggre- 

 gations of the tertiary and secondary centres arise true centrosomes 

 as new formations. Watase ('94-95) ^^^o finds in the q^^ of Mturo- 

 bdella, besides the normal aster containing an undoubted centrosome. 

 numerous smaller asters graduating downwards to such "tertiary 

 asters" as Reinke describes with a microsome at the centre of each, 

 and on this basis concludes that the true centrosome differs from a 

 microsome only in degree and may arise dc novo. iMottier ('97. 2) 

 finds in pollen-mother-cells numerous minute " cyto-asters " having 

 no direct relation to the spindle-formation (Fig. 133). Again Juel 



1 r/pp. 52. 214. 



