THE CENTROSOME 315 



The genesis of the concentric spheres surrounding the centrosome 

 will be considered in the following section. We may here only 

 emphasize the remarkable fact that the centres of the dividing 

 system arc bodies which are in many cases so small as to lie almost 

 at the hmits of microscopical vision, and which in the absence of the 

 surrounding structures could not be distinguished from other proto- 

 plasmic granules. Full weight should be given to this fact in every 

 estimate of the centrosome theory, and it is no less interesting in its 

 bearing upon the corpuscular theory of protoplasm. 



Watase ('93, '94) made the very interesting suggestion that the cen- 

 trosome is itself nothing other than a viicrosovie of the same morpho- 

 logical nature as those of the astral rays and the general meshwork, 

 differing from them only in size and in its peculiar powers.^ Despite 

 the vagueness of the word " microsome," which has no well-defined 

 meaning, Watase's suggestion is full of interest, indicating as it does 

 that the centrosome is morphologically comparable to other elemen- 

 tary bodies existing in the cytoplasmic structure, and which, minute 

 though they are, may have specific chemical and physiological prop- 

 erties. 



An interesting hypotliesis regarding the historical origin of centrosome is that of 

 Butschli ('91 ) and R. Hertwig ('92), who suggest that it may be a derivative of a 

 body comparable with the micro-nucleus of Infusoria, which has lost its chromatin 

 but retained the power of division : and the last-named author has suggested further 

 that the so-called " archoplasmic loops" discovered by Platner in pulmonates may 

 be remnants of the chromatic elements. A similar view has been advocated bv 

 Heidenhain ('93, '94) and Lauterborn ('96). Heidenhain regards central spindle 

 and centrosomes as forming essentially a unit ("microcentrum ") homologous with 

 the micro-nucleus of the Infusoria, the centrodesmus (p. 79) representing a part of 

 the original achromatic elements. The metazoan nucleus is compared to the proto- 

 zoan macro-nucleus. The improbability of a direct derivation of the Metazoa from 

 Infusoria, urged by Boveri ('95) and Hertwig ('96), has led Lauterborn ('96) to the 

 view that the metazoan centrosome and nucleus are respectively derivatives of two 

 equivalent nuclei, such as Schaudinn ('95) describes in Amceba biiiitcleata, the 

 " Nebenkbrper " of Paramceha {cf. p. 94), being regarded as an intermediate step, 

 and the micro-nucleus of Infusoria a side-branch. R. Hertwig ('96), on the other 

 hand, regards the metazoon centrosome as a derivative of an intra-nuclear body such 

 as the '• nucleolo-centrosome " of Euglena (p. 91), which has itself arisen through 

 a condensation of the general achromatic substance. With this view Calkins ('98), 

 on the whole, agrees; but he regards it as probable that the "nucleolo-centrosome" 



granules of which one or two remain as the persistent centrosome, while 'others are converted 

 into microsomes or other cytoplasmic structures. It is probable that something similar 

 occurs in the echinoderms. 



1 The microsome is conceived, if I understand Watase rightly, not as a permanent mor- 

 phological body, but as a temporary varicosity of the thread, which may lose its identity in 

 the thread and reappear when the thread contracts. The centrosome is in like manner not 

 a permanent organ like the nucleus, but a temporary body formed at the focus of the astral 

 rays. Once formed, however, it may long persist even after disappearance of the aster, and 

 serve as a centre of formation for a new aster. 



