io THE STRUCTURE OF CELLS 



granular inclusions. In plant cells centrosomes have been far less 

 often identified than in animals. They are more frequent, or at 

 least more easily demonstrable, in the lower members of the 

 vegetable kingdom than in the higher plants, in which they are 

 probably restricted to the motile sperms. The evidence for their 

 occurrence in angiosperms is not convincing. When present in a 

 cell, they usually occur in the form of a small granule enclosed in a 

 sphere, and are comparable with the centrosome and centriole of 

 Boveri. Strasburger has proposed the convenient term of centro- 

 sphere to designate the sphere together with its included granule, 

 reserving the term centrosome for the latter body only. It 

 is quite certain that the centrosphere apparatus presents itself 

 in varied degrees of complexity, not only in different organisms, 

 but even in different cells of the same tissues, and Strasburger's 

 term has much to recommend it, since, in spite of the large litera- 

 ture which has grown up around the subject, we are still mainly in 

 the dark as to the true meaning and relations of the different parts. 

 It seems clear, for example, that the centriole of Boveri corresponds 

 to that which by most writers has been called the centrosome, 

 and Boveri himself states that the division of his " centrosome " is 

 preceded by that of the centriole. 



The centrosome or centrosphere is itself not unfrequently 

 enclosed in a denser mass of protoplasm, called by Boveri the 

 Archoplasm, and by Strasburger the Kinoplasm. Probably, how- 

 ever, appearances denoted by these terms are not the expressions 

 of permanent structures, but represent transient phases of cellular 

 activity. The structures thus called into existence may, however, 

 be, at least temporarily, very pronounced, since at least a part of 

 the achromatic figure, which is formed during nuclear division, 

 owes its origin to the archoplasmic mass. Nevertheless, the archo- 

 plasm (or kinoplasm) may become absolutely indistinguishable at 

 other periods in the life of the cell. 



A far more difficult question to answer than that concerned 

 with the permanence of the archoplasmic or kinoplasmic structures 

 refers to the centrosome itself in a similar connection. Whilst 

 many authors have strenuously maintained its permanence from 

 one cell-generation to another, comparing it in this respect with the 

 nucleus itself, a considerable weight of negative evidence has never- 

 theless accumulated in the opposite scale. The striking relations 

 which obtain between the centrosome and the nuclear figures at 

 phases of division naturally produce a profound subjective impres- 

 sion upon the observer, and it has even been assumed that the 

 centrosome still persists, even when its actual existence cannot be 

 successfully demonstrated. There is no doubt that other granules 

 have frequently been mistaken for centrosomes, selected because 

 they happened to lie sufficiently near the spot where the structures 



