6i6 



SCIENCE PROGRESS 



another, and their relations to external heritable characteristics, 

 are very desirable. 



The two species of Protozoa that were chiefly used in my 

 investigations are shown in Fig. 7. They belong to the 

 class Rhizopoda and to the genus Arcella. Both species are 

 provided with shells secreted by their protoplasm. The 

 pseudopodia (which are locomotor and food-capturing pro- 

 trusions) are thrust out through a circular opening in the 

 centre of the under- side. The species Arcella dentata is char- 

 acterised by the presence of two nuclei, which can be seen 

 through the shell in the living animal, and by a circlet of spine- 

 like projections around the periphery which vary in number 



B 



Fig. 7. 



from 7 to 20. The diameter of the shell ranges from about 

 100 to 150 microns. Arcella polypora, the other species 

 studied, lacks these spines, but possesses a variable number 

 of nuclei ranging, in nature, from 3 to 13. The shell in this 

 species varies in diameter from 73 to 327 microns. Some of 

 the advantages of these organisms for nucleo-cytoplasmic 

 studies are as follows : they multiply rapidly by simple division ; 

 their morphological characteristics are definite and easily 

 measured ; they are large enough to be conspicuous when 

 placed under a binocular microscope ; they withstand severe 

 operations ; they can easily be cultivated ; and their nuclei 

 and other cell contents can be observed and measured through 

 the shell of the living animal. There is space available here 

 only for the statement of certain results of these investigations ; 

 but it is believed that the conclusions reached are based on 

 adequate data. 



Arcella dentata.— it was found that many strains exist 



