228 MEAD. [Vol. XIII. 



PAGE 



Mucous glands , 255 



Nervous system 256 



Problematic bodies 257 



Cilia 258 



Setse, parapodia, etc 259 



IV. Metamorphosis f^'om Free-swimming to Creeping Larvcz 260 



B. Clymenella torquata Verrill 261 



Habits and cleavage 261 



Summary 264 



C. Lepidonotus sp 265 



Habits and cleavage 265 



Influence of temperature, direct heat, and light 268 



D. ScoLECOLEPis viRiDis Verrill 270 



E. CHj^topterus pergamentaceus Cuvier 271 



Part II. — Comparative and General 275 



I. Homology of Cleavage Cells 277 



a. Equal cleavage 277 



b. Unequal cleavage 279 



Somatoblast 281 



32-cell stage, secondary trochoblasts 282 



" Stomatoblasts," 2S3 



Transition to 64-cell stage 284 



64-cell stage 284 



The " cross " 2S5 



Mesoblast 287 



Entoderm 290 



II. Cleavage considered from the Point of View of Developmental Mechanics 292 



Relative size of the cells 292 



Direction of cleavage 294 



Rate of cleavage 296 



III. Axial Relatio7is 299 



Bibliography 303 



Explanation of plates 305-326 



The material for the preparation of this paper was procured 

 at Woods Holl, Mass. The work has been carried on at the 

 Marine Biological Laboratory, at Clark University, and at the 

 University of Chicago under the direction of Dr. C. O. Whit- 

 man. I desire here to express my appreciation of the gener- 

 ous treatment which I have received at these institutions, and 

 especially to thank Dr. Whitman for his interest and friendly 

 counsel, which encouraged me to pursue these researches. 



