198 MAULSBY W. BLACKMAN. 



Protozoa, Calkins has this to say: " A distinct plasmosome or 

 true nucleolus comparable to the analogous structure in Metazoa 

 apparently exists in no case save possibly in Actinosphczrium, 

 and even here is limited to a passing phase during mitosis (Hert- 

 wig, '98). It is probable that the structures which have been 

 almost invariably but erroneously called nucleoli do not belong 

 at all to this category of nuclear elements but represent either 

 the functional chromatin which is aggregated into a central mass 

 (karyosome) during the quiescent or vegetative period of cell 

 life, or the intra-nu clear division center." From the work of 

 Griiber ('83), Rhumbler ('93), Labbe (96), Hertwig (98), Calkins 

 ('98, '01), and others, we must conclude that chromatin bodies 

 resembling nucleoli more or less closely are of very frequent 

 occurrence in unicellular animals. From Calkins' ('01) review 

 of these investigations it is evident that in its primitive condition 

 the chromatin is present in Protozoa in the form of dense homo- 

 geneous masses of chromatin (karyosomes) which act as the 

 nuclei of these undifferentiated cells. In higher types the nuclei 

 are more complicated. The chromatin may still occur in simple 

 masses, but these are contained within a nuclear membrane which 

 also encloses material other than chromatin (karyoplasm and 

 karyolymph). The spireme condition so characteristic of the 

 chromatin of metazoan germ cells is not commonly found in 

 Protozoa and when present, exists for only a short time. 



The karyosomes found in some of the higher types of proto- 

 zoan nuclei (Actinosphcerium, Hertwig) are not homogeneous 

 bodies of chromatin, but, besides this substance, also contain linin. 

 This linin often forms a reticulum upon which the chromatin is 

 deposited in the form of granules, an arrangement very similar to 

 that found in the nuclei of metozoan cells, and gives rise to a 

 structure which is similar to the chromatin reticulum of the more 

 differentiated nucleus. It is, however, still more strikingly like 

 the spireme structure of the karyosphere in appearance. That 

 it is different in some respects, however, is shown by comparing 

 the subsequent behavior of the two structures. The differences 

 are what would be expected when we take into consideration the 

 fact that one is contained in a protozoan cell while the other is in 

 a metazoan cell. The chromatin elements are much more firmly 



