i894. CELL-DIVISION. 45 



authorities that if any comparison can be made it is between the 

 centrosome and the micro-nucleus. Some recent researches, how- 

 ever, on the ova of vertebrates and arthropods have revealed the fact 

 that the centrosome of the ovum, although functionless, is relatively 

 of enormous size, and remains for a considerable time, even until the 

 embryo has reached an advanced stage in development. Later on it 

 disappears. If the macro-nucleus of a ciliate protozoon be looked 

 upon as a centrosome which, like the one just described, has lost its 

 power of setting up nuclear division, and is merely a rudimentary 

 organ, it can be homologised, or, at any rate, closely compared with 

 the centrosome of an ordinary ovum, which, as a rule, degenerates 

 and disappears. Hence, whether the centrosome be considered as a 

 nuclear organ or as belonging solely to the cell-body, it matters little ; 

 for it is easy to imagine the former case to be the more primitive 

 condition, and, from evidence gathered from different sources, we 

 are almost compelled to allow that both conditions actually do 

 occur. The following table may, therefore, be drawn up as 

 representing the probable phylogenetic history of the centrosome. 



(i.) A most primitive condition, where one nucleus only is 

 present, which seems to possess not only the hereditary chromatin 

 matter, but also power of spontaneous division, e.g., Amceha. 



(2.) A condition where the primitive nucleus becomes differen- 

 tiated into two ; one, the micro-nucleus, possessing the hereditary 

 substance, the other, corresponding to the macro-nucleus, which 

 functioned as an organ of division. Probably the two nuclei were of 

 more equal size than are the macro- and micro-nuclei. Unrepresented 

 as far as is yet known, 



(3). The macro-nucleus has dwindled to become a centrosome. 

 E.g., Spermatozoa, tissue cells, and ova where the centrosome is 

 practically active. 



(4.) The centrosome is rudimentary and disappears, to be 

 replaced by that brought in by the spermatozoon ; thus the primi- 

 tive condition of one nucleus is for a short time regained between the 

 giving off of the polar bodies and fertilisation. This nucleus, how- 

 ever, has not the power of spontaneous condition. E.g., inert ova. 



If these are the probable stages in the phylogenetic history of 

 the centrosome-, it will be seen that the condition obtaining in such 

 forms as Paramcecium is an offshoot of No. 2. The macro-nucleus, 

 after losing its function as a centrosome, has probably increased 

 largely in size and become a storage nucleus. The micro-nucleus 

 has taken on double duty, so to speak, i.e., it is not only the bearer of 

 hereditary substance, but is also capable of spontaneous division. 



It is by no means illegal to make this supposition of a change of 

 function for the macro- and micro-nuclei respectively, since such a phe- 

 nomenon is of constant occurrence in the animal kingdom. In the 

 afore-mentioned cases of vertebrate and arthropod ova, a most 

 primitive condition seems to exist. Although the macro-nucleus or 



