88 H. J. VAN CLEAVE. 



male and female producing strains are the factors specifically 

 considered by Shull in connection with his generalization that 

 there are "110 striking differences in the numbers of aberrant 

 glands under different circumstances." 



Serial sections were used by Shull in securing his evidence. 

 Numerous sources of probable error are inherent in data result- 

 ing from the study of serial sections. Practically all of these are 

 eliminated by the use of well prepared toto mounts. For many 

 features in a work as comprehensive as that of Martini whole 

 mounts would be entirely precluded for the study of finer de- 

 tails of minute structures, but in the study of gross structures 

 such as the vitellaria and gastric glands physical limitations do 

 not preclude the possibility of securing perfect data. 



VARIABILITY IN THE NUCLEI OF THE VITELLARIA. 



In the first 210 individuals examined by the writer each one had 

 8 nuclei in the vitellarium. At first this was considered as a fair 

 instance of random sampling offering conclusive evidence of ab- 

 solute constancy in the number of nuclei. Later, in an examina- 

 tion of 560 additional specimens, 3 individuals with aberrant num- 

 bers of nuclei in the vitellaria were discovered. Two> of these 

 vitellaria possessed 10 nuclei each and the remaining one 12. 

 Thus in 770 specimens examined less than 0.4 per cent, displayed 

 an abnormal number of nuclei in the vitellarium. Contrasting 

 these observations with those of Shull as shown in the following 

 table, two points are especially worthy of note: (i) No indi- 

 vidual with less than 8 nuclei in the vitellarium has been observed 

 by the present writer and (2) in no instance has an odd number 

 of nuclei been observed. 



TABLE I. 



FREQUENCY OF DIFFERENT NUMBERS OF NUCLEI OBSERVED IN THE VITELLARIA 



OF Hydatina senta. 



Number of nuclei 5 6 7 8 9 i o 1 1 12 



Times observed by Shull i o 5 235 3100 



Times observed by Van Cleave o o o 767 0201 



The plan of the arrangement of the nuclei in the vitellarium of 

 Hydatina is not immutable. There is, however, a strong ten- 



