CHROMOSOMES AND CYTOPLASM IN PROTOZOA 



51 



viduals of a very small race B, in which 

 the mean length of the individuals is but 

 73 microns. The relative sizes of the two 

 are shown in their correct proportions at 

 the upper left in Fig. 1. The large indi- 

 vidual has about 20 times the bulk of the 

 smaller one. The large and small sizes are 

 hereditary in the two races, so long as they 

 multiply separately. All individuals of 

 race A are large; all those of race B are 

 small. Two individuals of these races, dif- 

 fering greatly in hereditary size, conjugate 

 and exchange halves of their nuclei. Then 

 they separate. Each retains its own cyto- 

 plasm. They are now alike as to nuclei 

 and diverse as to their cytoplasm. They 

 are still very diverse in the sizes of their 

 cytoplasmic bodies. 



After separation the two individuals 

 divide in the usual way. The large indi- 

 vidual A divides into two which are ' of 

 course at first small, having but half the 

 volume of the parent. But these two grow, 

 becoming practically as large as their 

 parent A. Similarly, the small individual 

 B divides into two, which grow only to the 

 small size of their parent B. The two off- 

 spring produced by each of course divide 

 again, and such divisions continue daily 

 for generation after generation, each of the 

 two ex-conjugants producing a great num- 

 ber of descendants. 



At first, as we have seen, the offspring 

 of the large individual are large, and the 

 offspring of the small individual are small. 

 This difference lies entirely in the cyto- 

 plasmic bodies of the two, since after con- 

 jugation the two are alike in nuclei. Now, 

 as we follow successive generations of de- 

 scendants of the two, we find that a differ- 

 ence in size continues for many generations, 

 but that the differences slowly become less 

 (Fig. 1). The descendants of the small 

 individual B increase in size as generations 

 pass; the descendants of the large indi- 

 vidual A decrease in size. Fig. 1, drawn to 

 scale, represents the average sizes of the 

 descendants at intervals of two or three 

 generations. At the end of about 22 gen- 

 erations, requiring 22 days, the descendants 

 of the large and the small parents have 



reached about the same size. All the de- 

 scendants are now intermediate in size, 

 between the two original parents. At this 

 size they remain until there is another 

 conjugation. 



The results can be due only to the fact 

 that after conjugation the two sets are 

 alike in their nuclei. The nuclear con- 



FiG. 1. Paramecium caudatum. Changes in size 

 in consequence of the crossing of a large race A 

 and a small race B. At the upper left are shown 

 in their correct proportional sizes the two indi- 

 viduals A and B united in conjugation. In the 

 column headed A are seen (following downward) 

 the successive different sizes, at intervals of two 

 days, in the clone descended from the ex-conjugant 

 A. In the column headed B are shown at the same 

 intervals (following downward) the successive sizes 

 in the clone descended from the ex-conjugant B. 

 Figures drawn to scale from the dimensions and 

 curves of De Garis (1935). 



stitution gradually alters the cytoplasm, 

 bringing the size in both sets ultimately 

 to that which is characteristic for the 

 nuclear constitution. The nuclear consti- 

 tution finally dominates completely. But 

 for 22 generations the cytoplasmic differ- 

 ences affect the characteristics. 



Many such crosses between large and 

 small races were made by De Garis (1935). 

 In every case the cytoplasm affects the size 



