86 H. J. VAN CLEAVE. 



pared for class study. A preliminary examination of more than 

 100 individuals of Hydatina failed to reveal any deviation from 

 the usual numbers of nuclei in either the vitellarium or the gastric 

 glands. The same slides were used in a laboratory class in ad- 

 vanced invertebrate zoology composed chiefly of university jun- 

 iors and seniors. For six years each member of this class, averag- 

 ing about 15 students per year, was required to note and report 

 upon the numbers of nuclei found in the vitellaria of specimens 

 examined. The assignment has always been worded in such a 

 manner as to- place a premium upon the discovery of unusual 

 numbers of nuclei, without giving the students any previous hint 

 of the work on cell-constancy. There has never been a verified 

 instance of other than eight nuclei reported by members of these 

 classes. Rather frequently nine nuclei have been reported but in 

 every instance in which a check has been made the ninth nucleus 

 has been demonstrated to belong to the adjacent ovary and readily 

 distinguishable from the nuclei of the vitellarium. The data 

 included in these student observations have not been included in 

 the tabulated results presented later in this paper but comprise 

 a very considerable bulk of evidence of the infrequency of varia- 

 tions in the numbers of nuclei in the material under consideration. 



Stimulated by the publication of conflicting observations by 

 Shull, the writer has decided to bring together and to extend his 

 original observations on this phase of the problem of the nuclear 

 numbers in Hydatina. 



Much of the data available for the statistical study of range in 

 variability are due to the careful assistance of Miss Roselle 

 Karrer who examined 550 individuals of Hydatina and by means 

 of a mechanical stage recorded the location of all specimens which 

 displayed peculiarities of any sort. A critical examination of all 

 unusual specimens was made independently by Miss Karrer and 

 the writer, and in no instance was there found any ground for 

 disagreement in the interpretation of the evidence presented by 

 the specimen. 



MATERIALS. 



The materials upon which these observations are based comprise 

 a series of slides, each containing numerous stained, whole 



