90 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the case. However, the conditions existing in Protozoa have not been 

 generally known until quite recently, and in explaining the structures in 

 question a great deal must depend upon our knowledge of their phy- 

 logeny. From the results of studies on Protozoa we learn that the occur- 

 rence of karyosomes, or of karyospheres, from which chromosomes are 

 derived, is not an aberrant phenomenon in animal cells. On the con- 

 trary, these structures represent the primary condition in which chroma- 

 tin occurs. For this reason we logically should not ask an explanation 

 of their occurrence, but should rather inquire why the chromatin is ever 

 arranged in a different manner. Our problem is to explain why the 

 chromatin is arranged in a spireme rather than in a solid or granular 

 mass. In the Protozoa a condition approaching the spireme exists for 

 only a short time in the prophase (Hertwig, '99, Actinosphaerium ; Cal- 

 kins, '98 a , Xoctiluca), while in the resting condition of the nucleus the 

 chromatin is aggregated into one or more homogeneous or granular 

 masses. In regard to the phylogeny of the chromosomes, as well as of 

 other structures of the cell, sufficient knowledge is not yet possessed to 

 warrant drawing any general conclusions. No comprehensive phylo- 

 genetic study of animal cells has ever been undertaken, and until such a 

 study is accomplished, or until the series of facts accumulated by the 

 work upon isolated types has become more complete, conclusions with 

 any degree of certainty as to their accuracy cannot be drawn. Calkins's 

 work upon Protozoa and his comprehensive discussion of the work of 

 other investigators upon these unicellular animals has probably done 

 more toward this end than that of any other author with which I 

 am acquainted, but our knowledge of the relationships existing be- 

 tween the various parts of the cell in Protozoa and Metazoa is still 

 very insufficient. 



In Metazoa the aggregation of the chromatin into chromatin nucleoli or 

 karyospheres seems to occur only in cells which continue for a long period 

 without dividing. Thus they are much more common in egg cells dur- 

 ing the growth period than in any other type of cell. They are of uni- 

 versal occurrence in the growing spermatocytes of Chilopoda, which in 

 general characteristics, and in the duration of this growth period, resem- 

 ble very closely the egg cells. Thus, I believe, we may conclude in a 

 general way that the karyosphere is found only in cells which for a long 

 period remain quiescent, i. e. do not undergo division. This conclusion 

 holds good for all known cases where similar structures occur, somatic 

 cells as well as germ cells. 



