254 H. J. VAN CLEAVE 



INTRODUCTION 



1. Materials 



During the fall of 1909 the writer began a study of the mor- 

 phology of the Acanthocephala under the direction of Prof. 

 Henry B. Ward. It happened that the form first taken up for 

 study, Eorhynchus^gracilisentis (Van Cleave), displayed a marked 

 uniformity in the number and arrangement of the nuclei in certain 

 regions of the body. These observations opened the question of 

 how far these phenomena of constancy existed in the other tissues 

 and organs of the body. As additional species in the sariae genus 

 came to the writer's attention, the analysis was carried on in a 

 comparative manner until at length the present paper deals wi^ 

 the results obtained from the study of five species, namely; 

 Eo, gracilisentis (Van C. 1913), Eo. longirostris (Van C. 1913), 

 Eo. emydis (Leidy 1852), Eo. cylindratus (Van C. 1913), and 

 Eo. tenellus (Van C. 1913). Camera drawings of the probosces 

 (figs. 1, 21, 29, 32 and 35), individual hooks (figs. 2, 30, 33 and 36) 

 and embryos (figs. 3, 24, 34 and 37) of these species, are given in 

 order to show what extreme differences of diagnostic characters 

 may occur even though this study brings out the fact that in 

 many regions of the body each individual nucleus of one species 

 may be homologized directly with a nucleus in a member of a 

 distinctly different species. 



2. Cell constancy and its relation to cell lineage 



While studies in cell lineage have added invaluable support 

 to the idea of orderliness in vital processes they have failed to 

 carry this idea to its legitimate conclusion. The development 

 of the embryo through the cleavage stages of the ovum to the 

 formation of the germ layers and even up to the beginning of 

 differentiation in the anlagen of the organs of the adult body has 

 been worked out within various species of animals, but most in- 

 vestigators have dropped the problem here. The more or less reg- 



- The name Eorhynchus has been substituted for Neorhynchus, preoccupied, 

 in a paper now in press. 



