534 CELL MECHANICS )>^ 



/ 

 Sharp, 1920). In Drosophila it is probably, as a rule, too small to 



see (Dobzhansky, 1934). It may be represented by a diffuse 

 " centrosphere " or by a compact granule at the limit of visibility 

 {0.2 jx in diameter). In the polychaete Ophryotrocha the daughter 

 centrosome on the periphery of the cell disappears in the first divi- 

 sion of the oocyte, while the other remains visible and active. In 

 the Protista the centrosome is often associated with other permanent 

 cell-organs, whose exact relationship with those found in the higher 

 organisms is difficult to make out (Cleveland, 1934 and 1935). 

 Wherever centrosomes are found, with rare exceptions, they control 

 the formation of the spindle. They divide and during the prophase 

 the halves separate to opposite sides of the nucleus ; they develop a 

 radial structure in the cytoplasm around them and this structure 

 extends to form a spindle between them. 



Continuity in the inheritance and function of the centrosome has 

 been shown experimentally in many ways. It seems to be brought 

 into the egg by the sperm in animals, the egg losing its own centro- 

 some. In the parthenogenetic Rhahditis the sperm introduces into 

 the egg its own centrosome, which is invisible at every other stage, 

 although no fusion of nuclei, no true fertilisation, takes place. 

 Where several sperms can be induced to enter an echinoderm egg 

 their several centrosomes develop multipolar spindles (Boveri, 1907 

 etal.). Similar spindles arise in cells whose centrosomes have divided 

 but whose nuclear division has been suppressed by etherisation 

 (Wilson, 1928, p. 175), in fertilised egg-fragments of Triton , and in 

 anomalous mitoses in Vahlkampfia (cf. Table 74). In these 

 organisms, therefore, the centrosome is a self-propagating body 

 controlling the formation of the spindles. 



In the higher plants it is doubtful whether centrosomes ever 

 occur. They have been described by Guignard, Feng (1934) and 

 others in the germ cells, but Geitler (1934) has failed to confirm their 

 presence. The failure to find them may, as the studies elsewhere 

 show, merely indicate their smaller size or failure to react to the 

 system of fixation and staining used. Moreover, the more cylindrical 

 shape of the spindle in higher plants does not argue against single 

 pole-determinants, for such spindles are found with centrosomes in 

 Gregarina and Oligochaeta (Belar, 1928, p. 255). On the other hand ^ 



