68 KARYOKINESIS. 



What is the significance of this parallelism between the nucleus and the cen- 

 trosome ? Does it indicate that these two structures are genetically related or may 

 it be due to simpler physical or physiological factors? The parallelism in the 

 growth and diminution of these structures indicates that both nucleus and centro- 

 somes are diffusion centers which alternately enlarge by the absorption of substances 

 from the cell body and diminish by the return of substances to the cell body. It 

 is possible that their parallel changes in staining reactions are the results of the 

 absorption of similar materials from the cell body, or it may be clue to the fact that 

 the centrosomes imbibe a certain amount of material which escapes from the 

 nucleus. The fact that the}- are both self propagating is a property which they 

 share in common with other cell constituents (e.g. plastids) with which they are 

 certainly not homologous. 



On the other hand the remarkable manner of this self propagation is shared, I 

 believe, by no other cell constituents. Moreover, the singular resemblance between 

 the reticular central spindle (netrum), the achromatic reticulum within the nucleus 

 and the intranuclear spindle of many of the Protozoa is most striking and finds no 

 satisfactory explanation along the lines just indicated. 



Many persons who have worked upon nuclear division in the Protozoa (e.g. 

 Butschli ('91), R. Hertwig ('92, '99), Schaudinn ('95, '96), Lauterborn ('96), Calkins 

 ('98) have pointed out the resemblances between micro-nucleus or the intra-nuclear 

 mitotic spindle and the centrosome of the Metazoa, and this homology has been 

 maintained with great force by Heidenhain ('94) and Boveri ('01). 



R. Hertwig in particular has repeatedly advocated this homology. In Actino- 

 sphaerium he has observed that the centrosomes are actually budded out of the 

 nuclei, and he concludes that the centrosomes are to be considered an escaped achro- 

 matic substance of nuclear origin, "nuclei without chromatin." He has also pointed 

 out the steps by which, he thinks, the evolution of the centrosome has taken place, 

 as well as the phylogenetic relationships of the various types of centrosomes to each 

 other and to the nuclear structures of the Protozoa. 



Heidenhain also has forcibly presented the resemblance between the central 

 spindle of the Metazoa and the micro-nucleus of the Infusoria. He concludes that 

 the two are homologous, that the centrosomes of the Metazoa are only polar differ- 

 entiations of the intra-nuclear spindle of the Infusoria, while the macro-nucleus of 

 the latter corresponds to the nucleus of the Metazoa; the chromatic substance of 

 the micro-nucleus has disappeared in the Metazoa, being transformed into the archo- 

 plasm zone. 



These ideas of Heidenhain called forth the severe criticism of Boveri ('95) who 

 held that since the Infusoria cannot possibly represent the ancestors of the Metazoa, 

 the nuclear structures' and functions which occur in these cannot properly be con- 

 sidered the prototypes of those in the Metazoa. Further, he held that the macro- 

 nucleus was probably a transformed micro-nucleus, and that actual, independent 

 centrosomes were present in some Protozoa. 



It is interesting to find that in his recent work on the centrosome Boveri ('01), 



