ZOOLOGY AND BOTANY, MICKOSGOPY, ETC. 5l7 



spherical, and of a diameter equal to that of the thread-width common to 

 the sub-division. 



The author has constructed the following working hypothesis. The 

 chromatin granules of the simplest Protozoa are a visible expression of 

 differentiation and aggregation of specialized particles concerned with 

 the transmission of hereditary characters, and such granules probably do 

 not represent the sole " bearers of heredity " in the cell. The granules 

 become converted into rods by purely linear growth, accompanying 

 evolutionary development and greater complexity of the organism. This 

 rate of growth is not the Same in all chromosomes, and rods of various 

 lengths are therefore evolved. A stage is later reached in phylogeny 

 when rods have attained a maximum length, the limit depending upon 

 physical conditions, possibly connected' with spindle mechanism. When 

 this occurs, chromatin units conjugate in fours, and the resulting rods 

 have a diameter equal to twice that previously seen. These segment 

 later into spherical chromosomes of the new thread-width, and sucli 

 chromosomes are prepared to enter a new course of linear growth, 

 accompanying further evolutionary development. Thus the chromo- 

 somes of all phyla below Nematohelminthes have evolved. 



When the length limit has again been reached, conjugation once 

 more takes place, and rods are formed having a diameter equal to that 

 observed in Nematohelminthes and higher phyla. These rods later seg- 

 ment into spherical chromosomes of the new thread-width, and further 

 evolutionary development results in conversion of the latter to rods of 

 various lengths, such as are seen in complexes of Vertebrata. Thus in- 

 creased complexity of the organism is accompanied by increased 

 chromatin volume in the nucleus due to linear growth of granules or 

 spherical chromosomes, and the animal kingdom can be divided into 

 three groups, each representing a complete cycle of this process. 



The heterotropic or odd chromosome alone does not belong to the 

 general series, for its diameter exceeds the normal thread-width. Unless 

 it can be shown to be multiple, i.e. composed of several normal rods, such 

 as are found in association in certain organisms, we must assume that it 

 differs in every respect from the normal chromosomes. In any case, it 

 appears to be undergoing some process of development or disintegration 

 — probably the former — and may or may not be the determining factor 

 of sex. 



Significance of Chromatin in the Cell.* — Julius Schaxel discusses 

 the role of chromatin in the cell and sums it up in the word regulative. 

 He treats of the emission of chromatin from the nucleus into the cyto- 

 plasm and of the establishment of a chromidial apparatus. The 

 chromatin functions as a regulative substance, determining the construc- 

 tive processes that go on in the cytoplasm. 



Cells in Giant Tadpoles.!— A. Hahn has studied the cells of giant 

 larvae of Rana escuJenta, and confirms Levi's conclusion that epithelial 

 and glandular cells have the normal size, while ganglion cells are larger 



* Verb. VIII. Interuat. Zool. Kongress zu Graz., 1910 (1912) pp. 389-95 (5 figs.), 

 t Arch. Mikr. Anat. Ite Abt., Ixxx. (1912) pp. 1-38 (8 pis. and 18 figs.). 



