ARTICLES 623 



Hegner, R. W., Variation and Heredity during the Vegetative Repro- 

 duction of Arcella dentata, Proc. Nat. Acad. Sci., 4, 283-8, 1918. 



Hegner, R. W., Quantitative Relations between Chromatin and Cyto- 

 plasm in the Genus Arcella, with their Relations to External Char- 

 acters, Proc. Nat. Acad. Sci., 5, 19- 22 . 19190- 



Hegner, R. W., Heredity, Variation, and the Appearance of Diversities 

 during the Vegetative Reproduction of Arcella dentata, Genetics, 4, 

 95-150, 1919&. 



Hegner, R. W., Relations between Nuclear Number, Chromatin Mass, 

 Cytoplasmic Mass, and Shell Characteristics in Four Species of the 

 genus Arcella, Joum. Exper. Zool. 30, i-95» 1920. 



Hertwig, R., Encystierung und Kernvermehrung bei Arcella vulgaris, 

 Kupffer's Festschrift, p. 567, 1899. 



Hertwig, R„ Korrelation von Zell- und Kerngrosse, Biol. Centralb., 23, 

 49, 108, 1903. 



Jennings, H. S., Heredity, Variation, and Evolution in Protozoa, Proc. 

 Amer. Phil. Soc, 47, 393~54 6 < I 9o8. 



Jennings, H. S., Heredity, Variation, and the Results of Selection in the 

 Uniparental Reproduction of Difflugia corona, Genetics, 1, 407-534, 1916. 



Middleton, A. R., Heritable Variations and the Results of Selection in 

 the Fission Rate of Stylonychia pustulata, Joum. Exper. Zool., 19, 451- 



503. 1915- 

 Minot, C. S., The Problem of Age, Growth, and Death. New York, 1908. 



Root, F. M., Inheritance in the Asexual Reproduction of Centropyxis 



aculeata, Genetics, 3, 174-206, 1918. 

 Sachs, J., Beitrdge zur Zellentheorie. Flora, 1892. 

 Strasburger, E., Ueber die Wirkungssphare der Kerne und die Zellen- 



grosse, Hist. Beitr., 5, 1893- 

 Swarczewsky, B., Die Fortpflanzungserscheinungen bei Arcella vulgaris, 



Arch. f. Protist., 12, 173-2' 12, 1908. 



EXPLANATION OF FIGURES 



Fig. i. Stylonychia pustulata. Diagrams of successive fissions among the 



progeny of a single specimen illustrating the variations in fission rate. 

 The left side of the figure traces a series of " fast " selections, the 

 right side a series of " slow " selections, showing that at a given moment 

 we may have, among the progeny of a single parent, individuals of the 

 fourth and of the second generation. (From Middleton, 191 5.) 



Fig. 2 . Stylonychia pustulata. Curves indicating the difference in rate of 



fission between " fast " lines (continuous line) and " slow " lines 



(broken line). (From Middleton, 1915.) 

 Fig. 3. — Difflugia corona. Typical specimens from five different strains, 



showing the character of the diversities observed. (From Jennings, 



1916.) 

 Fig. 4. — Difflugia corona. Typical specimens from five heritably diverse 



branches of a single family, showing differences in size of body and 



length of spine. (From Jennings, 19 16.) 



