CELL CONSTANCY IN THE GENUS EORHYNCHUS 285 



as the animal becomes smaller, and that with a retm-n to normal 

 diet, when the animal grows very rapidly, its cells again increase 

 in volume." 



2. The heginning of constancy 



Throughout the entire animal kingdom observations, chiefly 

 incidental, have been recorded indicating that an organism or a 

 single organ contains a fixed number of cells or of nuclei. Orig- 

 inally this condition was looked upon as of uncommon occurrence. 

 Various workers among the protozoa have recorded the tendency 

 for a single individual during sporulation or colony formation to 

 give rise to a fixed number of cells. Thus in Tillina four spores 

 are usually formed from a single individual, but this condition has 

 not been absolutely fixed, for occasionally but two spores are pro- 

 duced. This would seem to indicate a transition from a repro- 

 duction by simple binary fission to a higher type where a 

 larger number of individuals is produced from the parent cell. 

 A step higher in the scale is Colpidium which, though normally 

 producing four individuals in its temporary cyst, not infre- 

 quently forms eight. Among the colonial protozoa a single 

 colony is frequently composed of a definite, constant number 

 of cells: as an example of this Gonium pectorale might be 

 cited. This is a sixteen cell colony in which each cell of the 

 association in turn gives rise to a sixteen cell aggregate while 

 still within the parent colony. In Eudorina is found an example 

 where thirty-two undifferentiated cells arise from a parent cell 

 producing a colony, as also in Pandorina which contains sixteen 

 undifferentiated cells associated together. Thus in the protozoa 

 there is a transition from the condition where reproduction is 

 accomplished solely by simple binary fission, through a series 

 of stages to that in which a definite number of cleavages result 

 in the formation of a colony having a fixed number of individuals. 



In the Metazoa, Conklin ('98) has distinguished a difference 

 between the earlier and the later cleavages, attributing to the 

 former the greater morphological importance: 



JOCRXAL OF MORPHOLOGY, \ OI,. 25, NO. 2 



