in the Animal Kingdom. 379 



approach of the end of the divisions ; nevertheless it must at 

 the same time be remarked that the amitotic division of the 

 niacronuclens runs a more regular course and stands much 

 nearer to mitotic division than the typical cases of amitotic 

 tlivision which occur in tlie iletazoa. 



It would not be quite correct simply to assert that in the 

 Protozoa direct nuclear division is followed by division of the 

 cell, because before a ciliated Tnfusorian or an Acinetarian 

 divides a double nuclear division takes place — the direct 

 division of the macronucleus and the indirect of the micro- 

 nucleus *. 



It follows from what has been stated that also in Protozoa 

 amitotic division, in so far as we know it at present, is seen 

 not as the primeval method, but as that which is of secondary 

 origin. AA'e have therefore now no empirical ground for the 

 view that indirect nuclear division has origiiuited phylogene- 

 tically from direct. The question as to the earliest origin of 

 mitosis leads to that of the earliest origin of the nucleus, and 

 is equally obscure, 



Freibnrjj: i. B., Zoolog-ical Institute of the Univer.sitv, 

 April 1891. 



Poi>tscript. 



A short time before I received the proof-sheets of this paper 

 there appeared M. Loewir's treatise on " Regeneration and 

 Constitution of the VVhite Blood-coipuscles " (" Neubildung 

 und Bcschaftenheit der weissen Blutkorperchen," Ziegler's 

 Beitrage zur pathol. Anatomic und allg. Pathologic, 10 Bd., 

 1891, p. 21o), in which it is stated that the cells which float 

 ill the blood of the crayfish always exhibit amitotic nuclear 

 division ; this nuclear division is frequently followed by 

 division of the cell, but multinuclear cells also occur. It 

 appears to me that no objection can be derived from these 

 observations against the statements which I have made above: 

 for, in the first place, Loewit himself gives a detailed descrip- 

 tion of the secretory nature of the cells of the crayfish's blood ; 

 he mentions that " in tlie cell-body of numerous cells of the 

 crayfish's blood in the fresh state glistening drop-like 

 structures of varying form and size, and resembling fat, are 



* Since iu many Aciuetaria, and especiailj in the swanu-spores of 

 Podophrya, micronuclei have been shown to exist i^vidv Biitschli, lot-, cit. 

 p. 1873; Maupas, Ijc. cit. p. 380), the well-known constricting-oif of the 

 nucleus in the fomiation of the swariu-sporcs of Podnphrija snnplv repre- 

 f-euls the division of the macronucleus. 



