blackman: the spermatogenesis of scolopendka. 83 



and Rhizopoda, the karyosomes of the nuclei of higher Protozoa, and the 

 karyosomes and karyospheres 1 found in the nuclei of Metazoa. 



In Metazoa. — Xucleolus-like bodies other than true plasmosomes have 

 been reported in the cells of a vast number of Metazoa. Structures ap- 

 parently composed of pure chromatin (karyosomes) are of very frequent 

 occurrence in tissue cells, and are especially characteristic of the large 

 ganglion cells of nervous tissue. In germ cells they occur much more 

 frequently in the female element than in the male. (The accessory 

 chromosome in the male cells must be included in a different class of 

 bodies, as it is a distinct element, a chromosome, as distinguished from 

 karyosomes, which are not such definite structures.) Indeed, in the 

 great majority of immature eggs the growth period preceding maturation 

 is as a general thing characterized by the appearance within the germi- 

 native vesicle of nuclear bodies, some of which are plasmosomes, while 

 many are plainly composed either entirely or in part of chromatin. In 

 many cases these chromatin bodies have been described as taking no part 

 in the subsequent activities of the nuclear structures. The chromosomes 

 are not derived from them, and the karyosome upon the disintegration of 

 the nuclear membrane is either set free in the cytoplasm, where it de- 

 generates (Wheeler, '95 ; Mead, '98 ; Griffin, '99, and others), or previous 

 to the maturation divisions is extruded from the germinative vesicle and 

 gives rise to yolk material (Balbiani, '93, et a?.). 



In many eggs in which the "nucleolus" behaves in this manner, it 

 is said to be derived by a chemical transformation, not from the chroma- 

 tin reticulum of the nucleus, but from other sources. However, in some 

 cases the authors assert that these bodies are formed from material de- 

 rived from the functional chromatin of the cell. The chromatin reticu- 

 lum within the nucleus, so thet affirm, breaks down and later asreresrates 

 to form a large karyosome. Yet this is later cast out into the cytoplasm 

 and no longer functions as chromatin. I believe it is very improbable 

 that chromatin which has once been a part of the chromosomes should 

 be eliminated in this manner without an accurate division such as occurs 

 at the time of the formation of the polar cells. Such reported cases 



1 In the use of the term ''karyosome " I limit it to structures found within the 

 nucleus which are composed exclusively of chromatin but are not discrete chromo- 

 somes. The karyosphere is much more complex, for it contains chromatin (in 

 granular, reticular, or spireme form), karvoplasm in the form of liuin, and karyo- 

 lymph ; it may also contain nucleolar material. It is, in fact, a miniature nucleus. 

 True nucleoli or plasmosomes come under an entirely different category, for kary- 

 osome and nucleolus have no real analogy with each other, the only points of 

 similarity being their intranuclear position and approximately spherical shape. 



