1904,] NATURAL SCIENCES OF PHILADELPHIA. 687 



THE EARLY DEVELOPMENT OF DINOPHILTJS: A STUDY IN CELL-LINEAGE. 



BY JAMES A, NELSON, PH.D., 

 University of Pennsylvania. 



Contents. 



I. Introductory and Historical 687 



II. Material and Methods 689 



III. Nomenclature 691 



IV. History of the Cleavage to the Formation of the Germ Layers 692 



(1) Unsegmented Ovum. 



(2) The Primary Cleavages. 



(3) The Segregation of the Ectohlast. 



(4) The Segregation of the Mesoblast. 



V. Outline of the P'urther Development 702 



VI. History of the First Quartette 705 



(1) The Cross and the Intermediate Girdle Cells. 



(2) The Prototroch. 



VII. Historv of the Second and Third Quartettes 714 



(1) M (X). 



(2) 2a, 2h and 2c and the Third Quartette. 



VIII. History of the Fourth and Fifth Quartettes 721 



(1) The Entomeres. 



(2) The Mesomeres. 



IX. Axial Relations. 724 



X. The Phylogenetic Relationships of Dinophilus in the Light pi its 



Early Development 727 



It is a pleasure to me to acknowledge my debt of gratitude to Prof. 

 E. G. Conklin, of the Zoological Laboratory of the University of 

 Pennsylvania, for his kindly advice and constant encouragement in 

 the preparation of this paper. I wish also to thank Prof. T. H. 

 Montgomery and Dr. J. P. Moore for many valuable suggestions 

 offered dviring the pursuance of the work. To Prof, C, 0. Whitman 

 I am indebted for courtesies extended me at Wood's Hole, Massa- 

 chusetts, In finishing the figures and in preparation of the manu- 

 script for publication, it gives me pleasure to acknowledge my 

 indebtedness to my wife for invaluable assistance. 



I, — Introductory and Historical, 



The phylogenetic relationships of Dinophilus have presented a per- 

 plexing problem to morphologists since the establisliment of the genus 

 by Oscar Schmidt in 1848. The various views as to its systematic 

 position may be conveniently classed under three heads: (1) those 

 referring Dinophilus to the Turbella ia; (2) those referring it to the 



