388 CALKINS. 



held together by a connecting rod of its own substance, the 

 chromatin granules begin to migrate towards the center of the 

 connecting rod and finally form a complete ring around it. 

 The Ncbenkorpcr is thus a central body similar to the central 

 body of Eiiglcna at a corresponding stage in division (cf. Figs. 

 17, 18, 19). After division of the chromatin the daughter-nu- 

 clei are reformed, but in each case the central body is left out in 

 the cytoplasm. This phenomenon recalls the conditions in Tc- 

 traniiUis where a similar protoplasmic body acts in a very similar 

 manner (cf Figs, i to 5). The latter form is more primitive 

 however, for the chromatin is not collected in a definite body — 

 the nucleus — but is distributed throughout the cell and collects 

 only during and for cell division. 



Protozoan Nuclei of Typical INIetazoan Structure. 



A typical metazoan nucleus differs from the forms described 

 above in having a distinct linin reticulum with chromatin laid 

 down within it and forming a chromatin reticulum ; often a 

 more or less clearly differentiated nucleolus, and a nuclear 

 membrane which usually disappears during mitosis. Many 

 important differences are found when a comparison is made 

 of the nuclei during division. In the majority of Metazoa 

 there is a distinct spirem leading up to the formation of chro- 

 mosomes in each case characteristic of the species ; and distinct 

 spindle-formation with centrosomes and spindle fibers. In the 

 Protozoa a number of nuclei have been described which agree 

 more or less closely with the requirements of such a nucleus. 

 So far as the resting nucleus goes, Actinosphceruiui and the 

 nuclei of some Sporozoa are similar to the Metazoan nuclei 

 w^hile the nuclei of ActinopJirys and of EnglypJia approach 

 them in mitosis. The number of such cases, however, is very 

 small and, when compared with the number showing the inter- 

 mediate type, it is insignificant. In short, the vast majority of 

 Protozoa excluding the infusoria possess various conditions of 

 nuclei of the intermediate type. 



