290 • H. J. VAN CLEAVE 



developed only in certain groups of animals? If the latter be the 

 case, then the question arises as to the factor or factors operating 

 in widely separated groups, which could accomplish such an end. 

 Up to the present time no direct attempt has been made to answer 

 this question. There seems to have been a tendency, at least 

 among some workers, to expect too great a future for the study of 

 cell constancy. .This has probably grown out of the failure to 

 realize fully the limitations of the field. There is the possibility 

 of a comparative anatomy, such as Martini has predicted, in 

 which two forms may be compared one with the other in a much 

 more intimate way than has been undertaken heretofore. In 

 this case individual cells rather than gross anatomical structures 

 would be used as units for comparison. An examination of the 

 facts brought out in the earlier parts of this paper will show that 

 they constitute just such a study in comparative anatomy. 

 Necessarily such a study is limited to those groups which have 

 acquired constancy. 



The fact that cell constancy has never been demonstrated in 

 any marked degree in organisms which hold an unquestioned 

 position in the main line of descent of the animal series seems to 

 indicate that it is not the primitive condition. The Nematodes, 

 the Tunicates, the Rotifers, and the Acanthocephala, forms 

 which most clearly display the phenomena of constancy, stand 

 either as side branches from the main line of descent of animal life 

 or as highly modified groups which have lost most of their indi- 

 cations of close relationship with other forms. For instance the 

 rotifers have reached a point in development but little higher 

 than the trochophore stage found in the development of higher 

 forms and have become permanently fixed. The tunicates, com- 

 ing off from the stem which has given rise to the Vertebrates, have, 

 in a similar manner, either failed to proceed farther or, in case at 

 one time they stood on a higher level, have through degeneration 

 regressed to a condition where they have become fixed as an 

 aberrant group. Parasitism has so profoundly affected the somatic 

 portions of the Acanthocephala that little is left to serve as an 

 indication of earlier phylogenetic relations. Especially, in this 

 group, species and even genera differ from each other in slight 



