THE 

 CYTOGENY OF PODARKE OBSCURA VERRILL. 



AARON L. TREADWELL. 



CONTENTS. 



Part I. — Descriptive. page 



Introductory 399 



Egg-Laying and Methods 400 



One to Two Cells 404 



Two to Four Cells 404 



Four to Eight Cells 406 



Eight to Sixteen Cells 407 



Sixteen to Thirty-two Cells 407 



Thirty-two to Sixty-four Cells 408 



First Group of Ectomeres 412 



Second Group of Ectomeres 419 



Third Group of Ectomeres 425 



Larval Mesoblast 427 



Fourth Group of Ectomeres 429 



Fifth Group of Ectomeres 433 



Trochophore 434 



Part IL — General Considerations. 



Axial Relations 438 



Types of Cleavage 438 



Cell Mechanics 440 



Equal and Unequal Cleavage 441 



Mesoblast and Larval Mesoblast 449 



Cell and Regional Homology 455 



The following investigation, begun in 1891, has been carried 

 on at the Marine Biological Laboratory at Woods Holl, Mass., 

 in the Zoological Laboratory of the University of Chicago, and 

 in the intervals of teaching at Miami University. For many 

 helpful suggestions I am indebted to my friends, Drs. C. M. 

 Child, E. G. Conklin, and A. D. Mead. I desire especially 

 to acknowledge my indebtedness to Professor C. O. Whitman, 

 without whose friendly encouragement the work would hardly 



399 



