82 bulletin: museum of comparative zoology. 



Zacharias ('85, '87) den Ausdruck, Plastin gebraucht haben." During 

 the formation of the first polar cell, all the chromatin being withdrawn 

 from this structure, a true plasmosome nucleolus is produced. How- 

 ever, upon the reconstruction of the nucleus the chromatin is again 

 deposited in this body, and it resumes its wonted characteristics. This 

 structure corresponds to the " nucleoles mixtes " of Carnoy ('85). In 

 mitosis it breaks up into granules, which arrange themselves in .parallel 

 lines in the nucleus, resembling closely the chromatin segments of 

 metazoan cells. In the next higher type, represented by Noctiluca, the 

 chromatin is in the form of a number of karyosomes (generally 10—12) 

 which, during the prophase, behave in a manner similar to the single 

 one in Actinosphaeriurn ; but there is no residue of plastin. The 

 bodies found in the other types of nuclei do not appear to bear such a 

 close relation to the structure in question and therefore need not be 

 discussed here. 



From these results I believe we may conclude that the lower type of 

 nuclei found in the Protozoa is very similar in structure to the kary- 

 osome or karyosomes found in the more differentiated nucleus of this 

 group and in the nucleus of many Metazoa. Indeed, they seem to be 

 more nearly analogous to karyosomes than to the higher type of nuclei. 

 The karyosomes found in some of the higher types of nuclei among 

 Protozoa are not homogeneous masses of chromatin, but also contain, 

 besides this substance, linin. The liuin often forms a reticulum, upon 

 which the chromatin is deposited in the form of granules. This arrange- 

 ment is similar to that found in many of the nuclei in Metazoa, and 

 gives rise to a structure which is similar to the chromatin reticulum of 

 more differentiated nuclei. 



It is, however, in appearance still more strikingly like the spireme 

 structure of the karyosphere. That it is different from this, however, 

 in some respects, is shown by comparing the subsequent behavior of the 

 two structures ; but this difference is one that would be expected when 

 we take into consideration the difference in source. The chromatin ele- 

 ments are much more firmly established in the higher animals, and hence 

 it is to be expected that when the karyosphere breaks down, the resulting 

 fragments will be distinct chromosomes. In Protozoa the conditions are 

 different. The chromosomes are not such definite structures, and hence 

 when the karyosome disintegrates it gives rise to a large number of 

 granules, which later aggregate into chromosome-like masses. How- 

 ever, I believe the relationship is sufficiently close to warrant our placing 

 in the same general category the solid chromatin nuclei of some Sporozoa 



