454 THE AMERICAN NATURALIST [Vol. XLVI 



rate — i. e., producing but one or two embryos ; but such 

 instances were always due to degeneration or growth un- 

 der unfavorable conditions. I may add by way of con- 

 trast that the humped form most typically produces but 

 one young at a time. Thus, in the material of my orig- 

 inal study, among one thousand individuals I found but 

 sixteen showing the more or less simultaneous develop- 

 ment of two embryos, and none showed more than two. 

 The campanulates regularly develop several embryos at 

 once, four and five being common numbers ; but in their 

 capacious bodies there is no overcrowding, and certainly 

 no curtailment of nutrition. 



As to the occurrence, the definite relationship, and 

 the causes of the production of the three types of the 

 species, I have ascertained most of the facts, though 

 not all. 



The saccate fo*m is, I think, unquestionably the only 

 form that emerges from the resting egg. In all cases — 

 seven in number — in which I have examined temporary 

 ponds within a few days after formation and found this 

 rotifer just appearing, or in which I have caught the spe- 

 cies evidently close to its first appearance, I have found 

 either nothing but the saccate type, or numerous saccates, 

 together with transitional and humped types. In the 

 majority of these cases the saccates have been rapidly 

 displaced by the humped type, which, in nature at least, 

 very rarely reverts to the production of the smaller form. 

 As soon as the species is in full swing, so to speak, count- 

 less numbers may be examined for weeks without a sac- 

 cate occurring. All of these phenomena have been par- 

 alleled in my aquaria, and I have also repeatedly observed 

 the birth of the humped type from the saccate parent. 

 This is usually in instances where the high numbers of de- 

 veloping embryos in the saccate parent have given place 

 to few or to but one. Usually, too, the parent is a rather 

 large saccate and the humped young a rather small ex- 

 ample of its type. But occasional instances have been 

 noted where the humped young, a moment after birth or 



