. The cell in development and inheritance. Cells. 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 th
. The cell in development and inheritance. Cells. 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
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