ARTICLES 617 



within this species in nature. For example, from one specimen 

 with 10 spines, 148 descendants were obtained by fission ; 

 these had a spine number ranging from 8 to 14, and a mean 

 spine number of 1 1-04. By way of contrast, another specimen 

 was the progenitor of 39 descendants which varied in spine 

 number from 12 to 16, and had a mean spine number of 14-07. 

 Extensive measurements revealed the fact that a definite 

 size, as indicated by the diameter of the shell, is a very 

 constant character of the specimens that have descended 

 from one progenitor. Furthermore, when spine number and 

 diameter of shell were compared, it was discovered that these 

 two characters are very closely correlated, and that the greater 

 the diameter the larger the number of spines. Thus specimens 

 with 10 spines had a mean diameter of 112 microns; those 

 with 11 spines had a mean diameter of 114 microns; those 

 with 12 spines had a mean diameter of 118 microns, etc. A 

 large family containing 5,557 specimens and representing 69 

 generations was reared in the laboratory from a single specimen 

 by vegetative reproduction (fission). Selection as regards 

 spine number and diameter was practised within this family 

 by preserving on the one hand those specimens that had few 

 spines and were small in diameter, and on the other hand those 

 specimens that had many spines and were large in diameter. 

 By this means two heritably distinct branches were isolated 

 within this family ; the members of the larger branch had a 

 mean spine number of 11-48, whereas those of the smaller 

 branch had a mean spine number of 10-32 ; and these differ- 

 ences persisted for many generations after selection was dis- 

 continued (Fig. 8). It was concluded from these data that 

 the diversity obtained within this family represented a change 

 in the germ-plasm, and that evolution had thus been observed 

 in these small organisms in the laboratory. 



These measurements prepared the way for experimental 

 studies of nucleus and cytoplasm. As shown in Fig. 7, 

 Arcella dentata possesses two nuclei. These nuclei are almost 

 invariably located on opposite sides of the mouth opening as 

 indicated. They are of the vesicular type, the chromatin 

 being concentrated in the centre of the nucleus as a spherical 

 mass. Both the nuclei and the chromatin mass within them 

 were observed and measured while the specimens were alive. 

 The ability of these organisms to withstand severe operations 

 is quite remarkable, and many cutting experiments were 

 performed with them. Specimens, when cut into two or even 

 four pieces, continued to live and reproduce, and the part of 

 the shell that was cut away was in no case regenerated. When 

 a binucleate parent is cut in two so that each half contains 

 a single nucleus, the halves produce uninucleate descendants 



