380 CALKINS. 



may have arisen from simpler forms, and how in its mitotic 

 phenomena it passes through stages represented by permanent 

 nuclei of lower Protozoa. 



In the first comparison of metazoan with protozoan nuclei 

 we are at once beset with difficulties. Protozoan nuclei vary 

 so widely among themselves that, save for the same class of or- 

 ganisms, a description of one nucleus would not correspond at 

 all to that of another. Some resemble the ordinary type of 

 nucleus in the Metazoa, others are so different from this type 

 that they €an scarcely be compared. In general the nuclei of the 

 Protozoa are much simpler in structure than those of Metazoa. 

 Chromatin is present in all cases but other parts which are usu- 

 ally found in nuclei of the Metazoa are frequently missing, c. g. 

 the linin, the nuclear membrane or the nucleolus. On the 

 other hand, bodies are occasionally found within the nucleus of 

 Protozoa which are absent altogether or present in some other 

 form, in Metazoa ; such for example are the significant centro- 

 some-like bodies found in Eiiglcna and allied forms. 



There are so many different types of nuclei in the various 

 classes and orders of the Protozoa that it should be possible to 

 select a chain of forms connecting the simplest known type with 

 the highest. Such a sequence may be sought for in the struc- 

 ture of the resting nucleus or \\\ the method of division. An 

 ideal sequence would result if the two lines could be developed 

 simultaneously, but this is extremely difficult as a nucleus may 

 be high in the series of nuclear structures and low in the matter 

 of mitotic division. The nuclei o{ ActinospJui^riuiii, Actinop/irys 

 and Xoctihica offer a striking example of this fact, the two 

 former resembling the structure of the metazoan type more 

 closely than the latter, while in mitosis the latter is much nearer 

 the metazoan type than are the former. In questions of phyl- 

 ogeny however, morphological characters are usually of more 

 importance than physiological characters and this must be kept 

 in mind in the present discussion. A number of authors have 

 built up theories of phylogeny on the method by which the 

 nuclei of Protozoa divide, and the obvious result is a series of 

 mitoses which satisfy to a certain extent the requirements in 



