ARTICLES 621 



The measurements (see both preceding tables) show, for 

 example, that, as a rule, specimens that are about 112 microns 

 in diameter possess 4 nuclei in Family 5, and 7 nuclei in Family 

 34, but that the quantity of chromatin in each nucleus of the 

 specimens in Family 5 is greater than that in each nucleus of 

 the specimens in Family 34, and that the total quantity of chro- 

 matin in such specimens in Family 5 is approximately the 

 same as in those in Family 34. The conclusion is inevitable 

 that in these organisms each nucleus is associated with a rather 

 definite quantity of cytoplasm, and that the quantity of cyto- 

 plasm varies directly as the quantity of chromatin within the 

 nucleus. Why the chromatin masses should be smaller and 

 more numerous in specimens of one family (34) than in those 

 of the other family (5) is a problem still unsolved. 



Senescence and Death in Arcella dentata. — During the course 

 of the experiments with Arcella dentata, certain data were 

 obtained that seem to favour Minot's theory, that senescence 

 and death are due to an excess in the amount of cytoplasm as 

 compared with that of the nucleus. From a line containing 

 relatively small specimens (Fig. 8, ed) — specimens averaging 

 10 1 microns in diameter and 9-91 in spine number — a branch 

 line (Fig. 8, edb) was derived, consisting of enormously large 

 specimens averaging 153 microns in diameter, and 17-54 in 

 spine number. These large specimens received particular 

 attention, but in spite of every care they all died after several 

 generations. In the meantime, measurements of their chromatin 

 masses were made, and these were found to be almost exactly 

 equal in size to those in the small specimens from which they 

 were derived. Death in this case may have been due to the 

 failure of the chromatin masses to increase along with the cyto- 

 plasm, and hence due to an excess of cytoplasm over chromatin. 



Chromidia in Arcella. — Arcella is one of the first organisms 

 in which extra nuclear granules, now known as chromidia, 

 were discovered. A band of these granules was described 

 by R. Hertwig in 1887, and the same investigator twelve 

 years later (Hertwig, 1899) recorded the formation of secondary 

 nuclei from this " extra-nuclear chromatin net." These nuclei, 

 according to Elpatiewsky (1907), become the centres of amebulae 

 of two sizes, which are macrogametes and microgametes, 

 and which conjugate in pairs. The conjugation of the entire 

 chromidial nets of pairs of Arcellas was reported the following 

 year by Swarczewsky (1908). Chromidia have been described 

 in many groups of Protozoa, and seem to play an important 

 part in their life-cycle. For this reason it seems worth while 

 to mention the apparent lack of influence of this extra-nuclear 

 chromatin during the bisection experiments performed on 

 Arcella dentata. These experiments demonstrate that, when 



