No. 548] A CASE OF POLYMORPHISM 447 



when dealing with the earlier embryos, in rejecting any 

 case which was not quite certain. In other words, had I 

 been working with live material and rearing the animals 

 to a proper stage, a considerably larger proportion than 

 one fourth would probably have turned out heterotypic. 



To answer the second question — i. e., whether this 

 atypical reproduction were occurring in both directions 

 or in one direction only— I first made a preliminary 

 examination of several thousand individuals of the 

 humped form, a high percentage of which contained well- 

 developed embryos. I found no indication of any de- 

 parture from normal reproduction. I then examined 

 critically one thousand individuals, recording results in 

 each case. Of this one thousand individuals, 419 con- 

 tained young sufficiently developed and well placed to 

 admit of safe determination. All were plainly of the 

 parental type; nor did I find a single case in all this 

 material that even suggested strong variation toward the 

 larger humpless rotifer. 



Transitional specimens, however, could, if present, be 

 more certainly found among adults, or at least among in- 

 dividuals after birth. This was my third query. Did 

 such exist? Throughout my entire examination I sought 

 for them, but with little success. At certain stages in 

 development each species approaches a little nearer to 

 the other species. Thus the young of the larger, cam- 

 panulate type are not only smaller, but have slightly 

 bulging sides, suggesting the possibility of humps. 

 Their corona, too, although very heavy and broad, is less 

 disproportionately so than in the fully grown adult. But 

 in no case was it difficult to determine the relationship of 

 such specimens; other characters placed them at once, 

 especially the enormous and differently formed trophi, 

 which are fully developed before birth. Very old adults 

 of the smaller humped type might possibly, at first glance, 

 be taken for variants toward the larger form, in that 

 their humps become smaller and less sharply marked off 

 from the general body wall. In no other character, how- 



