J 36 
MURIEL ROBERTSON. 
gical literature means a structure composed of chromatin 
embedded in an achromatic substance, and which usually 
contains a centriole or centrosome. In general cytological 
writing the term is applied to a condensation of chromatin as 
distinguished from a true nucleolus. In neither sense are the 
authors justified in saying that the process they describe is 
really a heteropolar mitosis of the karyosome. The term 
“ mitosis ” implies some kind of a partition of all the sub- 
stance of the structure involved, and heteropolar mitosis of 
the karyosome means that the plastin, chromatin, etc., have 
undergone an unequal division. 
Wliat these authors show in their excellent work on the 
two species of Spongiomonas and on Cercomonas parva 
described in the same paper is the splitting-off of a minute 
centrosome-like granule which is bound permanently (as in 
Cercomonas), or for a time (as in Spongiomonas), to the 
karyosome by a slender thread. The centriolar nature of 
these granules is abundantly demonstrated in the last sentence 
of the paragraph quoted above. It is clear that this process 
is essentially the same as that described in the collar-cells, 
and brings the blepharoplast of these Protozoa into line with 
those of the sponge-cells. 
The main points raised in all this work are very clearly and 
broadly put in a valuable article (3) by Hertwig, who comes to the 
conclusion that the cytoplasm is a compound substance com- 
posed (a) of a substance very closely akin and practically identi- 
cal with the achromatic contents of the nucleus, and (b) of a 
substance akin to chromatin, and from which this latter is built 
up. Accordingto Hertwig’ s idea, the chromatic and achromatic 
substances are distinct and separated out from each other in 
the nucleus, while in the cytoplasm the same two substances 
are present in some sort of combination. The centrosome is 
for Hertwig simply “ein individualisiertes Stiickchen achro- 
matischer Kernsubstanz,” and adds that he is prepared to 
admit that centrosomes may arise outside the nucleus from 
the achromatic substance of the cytoplasm. These con- 
ceptions of Hertwig’s are of course to be considered as a 
