112 ■ TWENTY-FIRST REPORT. 



Preparations of cells made during the time of depression show the vege- 

 tative nuclei inactive or breaking up, and "secondary" nuclei forming from 

 part of the chromidial net. These secondary nuclei have been observed to 

 enter daughter cells, while the old vegetative nuclei remain in the old shell, 

 in many cases. Preparations of conjugating individuals show remarkably 

 similar conditions. 



These so-called "depression" periods are not a necessary event in the life 

 of the organism, since the reconstruction of nuclei can take place without any 

 very noticeable "depression" period. 



A "Uninucleated" race, that is, a line derived from a uninucleate indi- 

 vidual, was maintained through periods corresponding to the periods referred 

 to above. Some individuals with two nuclei occurred as offspring in this line 

 also. Thus the so-called vegetative nuclei are not necessarily organically con- 

 nected, but are merely in association. 



A three nucleated race was also maintained from May 13th to August 

 19th, 1918. The individuals of this "line" have a larger average size than 

 those of the "uninucleated" line, but the number of nuclei in an individual does 

 not necessarily determine its size, since many three nucleated individuals we 

 found smaller in size than many of the larger nucleated individuals. Indi- 

 viduals with two and with even one nucleus occurred as offspring in the three 

 nucleated line. These occurred in perfectly healthy cultures, and are not to 

 be assigned to cultural conditions. 



If the nuclei constitute the germplasm, its continuity is interrupted at the 

 time the old vegetative nuclei cease to divide and begin to disintegrate, and 

 the secondary nuclei are organized from the chromidial net and pass into 

 the new daughter cells. If chromidiogamy occurs at the time of conjugation, 

 as is maintained, nuclear continuity is still interrupted, and is only secured 

 in the process of the construction of new nuclei out of the chromidial net. 

 There is thus a discontinuity of nuclei with a nuclear cycle of development. 

 If the nuclei are successively similar at corresponding stages, the lines run 

 true; if not, variations due to their dissimilarities are produced. 



Arcella vulgaris is sensitive to environmental conditions, and all possible 

 care was taken to eliminate such influences. While it may not be possible to 

 entirely eliminate these, the data upon which these conclusions are based are 

 derived only from normal, clean, healthy cultures. Cultures showing unfa- 

 vorable conditions were discarded. The cultures were started in the latter part 

 of September, 1917, with specimens taken from a small pond on the campus 

 of Johns Hopkins University, and the experiments were carried forward in the 

 Zoological Laboratories of the University until the 2Sth of June. 191S, when 

 they were transferred to the laboratories of the Brooklyn Institute of Arts and 

 Sciences, Cold Spring Harbor, Long Island, New York. There they were 

 continued imtil August 19th, 1918. I am especially indebted to Professor 



