THE CENTROSOME 



225 



The earlier observers of the centrosome always found it lying in the cytoplasm, 

 outside the nucleus. Almost simultaneously, in 1893, three investigators indepen- 

 dently discovered it inside the nucleus of the resting cell, — Wasielewsky, in the 

 vouno- ovarian eggs (oogonia) of Ascaris ; Brauer. in the spermatocytes of the same 

 animal; and Karsten. in the cells of a plant. Psilotitm (Humphrey states, however, 

 that Karsten's observations were erroneous). Several later observers have described 

 a similar intra-nuclear origin of the centrosome, and several of these (Zimmermann. 

 Lavdovsky, Knuten) have followed Wasielewsky in locating it in tlie nucleolus. 

 Evidence against this latter view has been brought forward, especially by Humphrey 

 and Brauer. The latter observer found both nucleoli and centrosome as separate 

 bodies within the nucleus. He made further the interesting discovery that in 



t c 



^-^ — // 



A 



♦ 



B 



C 



D 



E 





^^^'^ 



a 



Fig. 107. — Mitosis with intra-nuclear centrosome, in the spermatocytes of Ascaris megalo- 

 ccphala, var. univalciis. [BraUKR.] 



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 figure, 

 ccntrosomes escaping from the nucleus in G. 



Ascaris the centrosome lies, in one variety {mtivalcns) inside the nucleus, in the other 

 variety {bivalens') outside — z. fact which proves that its position is non-essential 

 (cf. Figs. 92 and 107). Oscar and Richard Hertwig maintain that the intra-nuclear 

 position of the centrosome is the more primitive, the centrosome having been 

 originally differentiated from a part of the nuclear substance. This view is based 

 in the main on tlie facts of mitosis in the Infusoria, where the whole mitotic tigure 

 appears to arise within the nuclear membrane (cf. p. 62). 



Whether a true centrosome may ever arise de novo is Hkewise 

 undetermined. The possibiHty of such an origin has been conceded 

 by a number of recent writers, among them Biirger, Watase, Richard 

 Hertwig, Heidenhain, and Reinke. The latter author ('94) would 



Q 



