T. M. SONNEBORN 177 



tionably exist, but much more needs to be known before this 

 could become of much value in identification. 



Morphological, Anatomical, and Cytological Differences. The 

 most obvious morphological features are of course size and shape. 

 Unfortunately, however, these vary with cultural conditions, 

 stage of the division cycle, stage of the life cycle from one fer- 

 tilization to the next, and so on. Nevertheless, under comparable 

 conditions there are some striking differences, particularly in 

 length and breadth, among the varieties. Some unpublished ob- 

 servations by Sonneborn and Dippell, made in 1942, may be com- 

 pared with unpublished observations by Elizabeth Powelson 

 made 14 years later on different strains of the same varieties. 

 Only the mean lengths of overfed vegetative animals are given 

 here : 



Variety l Variety 2 Variety 4 

 Sonneborn and Dippell (1942) 139 M 145 m 112 n 



Powelson (1956) 131 m 149 m 112 m 



This remarkable agreement inspires much confidence in the 

 existence of very real size differences among the varieties, so 

 that others which were made at only one time may be added: 

 Variety 3, 123 n; Variety 7, 140 n; Variety 12, 204 fi. The meas- 

 urements on variety 12 are by Powelson, those on varieties 3 and 

 7 by Sonneborn and Dippell. Measurements on other varieties 

 are not available, but long familiarity with the material permits 

 me to estimate that the characteristic size in varieties 8, 10, and 

 14 is essentially the same as in variety 4, i.e., about 112 n. Like- 

 wise, the animals of varieties 13 and 15 and, judging by Giese's 

 report, 16, are very large, surely 170 m or more. The other varieties 

 are intermediate and roughly in the range 125 to 150 ^. This 

 leaves open the question of strain differences within a variety. 

 Such differences doubtless exist, at least in some varieties, but on 

 the whole they probably never blur the distinctness of the three 

 major varietal size classes listed above. The latter provide an 

 easy means of at once placing an unknown strain in one of three 

 groups of varieties. No measurements are necessary to do that, 

 for the differences are considerable to a practiced eye. 



