734 CALKINS. [Vol. XV. 



R. Hertvvig ('96) doubts Ishikawa's account of the centrosome 

 in Noctiluca, and, in accordance with his theory regarding this 

 body, he would probably consider the sphere in Noctiluca as 

 an enlarged centrosome. He also regards the centrosphere in 

 the sea-urchin egg as an enlarged centrosome, in opposition to 

 Boveri ('95), Hill ('95), and Kostanecki ('96), who have found cen- 

 trosomes within it.i From his experiments with strychnine on 

 developing eggs he also concludes that the centrosome may 

 have the form of pole-plates, as in some Protozoa. He supposes 

 the centrosome to be, originally, an intra-nuclear structure (as 

 in Englena, Spirochona, etc.), which becomes extra-nuclear in 

 Noctihica, Paramoeba, and the majority of Metazoa. The intra- 

 nuclear pole-plates of most Protozoa, the sphere in Noctiluca 

 and Paramoeba, and the sphere in Echinoderm eggs are, there- 

 fore, regarded by Hertwig as homologous structures, each of 

 which he would call the centrosome. At first sight this is a 

 most plausible theory, but many indubitable facts stand in its 

 way. Boveri, Hill, and Kostanecki have found centrosomes in 

 the sphere of sea-urchin eggs; Balbiani ('95) has described less 

 substantially a centrosome in the pole-plate of Spirochona ; and 

 Ishikawa demonstrated the centrosome within the sphere of 

 Noctihica, an observation which I can fully confirm.^ 



As there is no trace of the centrosome in the sphere during 

 nuclear rest, the conclusion is apparent that its presence here 

 during nuclear activity can be accounted for either by forma- 

 tion de novo or by migration from some other part of the cell. 

 This alternative involves a question in cellular biology which 

 is far from settled — viz., is the centrosome a permanent ele- 

 ment of the cell .' 



In the first place, a number of investigators, especially 

 among botanists, deny entirely the presence of a centrosome 

 in certain dividing cells (Strasburger ('97), Osterhout ('97), 

 Mottier ('97), etc.), while the remarkable observation made by 

 Juel ('97) seems to indicate that the centrosome is not neces- 

 sary in the formation of a spindle-figure. Juel found that a 

 single chromosome was parted from its fellows and formed a 



1 Wilson also ('97) finds unmistakable centrosomes in Toxopneustes and Arbacia, 

 which are derived from the middlepiece of the spermatozoa. 



2 See appendix, p. 49. 



