Volume VI. July, i8g2 Number j. 



JOURNAL 



MORPHOLOGY 



THE CELL-LINEAGE OF NEREIS. 



A CONTRIBUTION TO THE CYTOGENY OF THE ANNELID 



BODY. 



EDMUND B. WILSON. 



TABLE OF CONTENTS. 



Page 

 Introduction 362 



I. The unsegmented ovum 375 



II, General sketch of the development 377 



III. Spiral period of cleavage 383 



1. The first cleavage 383 



2. The second cleavage 385 



3. The third cleavage 'i^'J 



4. The fourth cleavage 388 



5. The fifth cleavage 390 



6. (a) Third spiral cleavage of the primary micromeres ; Qa) for- 



mation of the apical rosette ; origin of the second somatoblast ; 

 (c) second division of the first somatoblast 392 



IV. Transition to the bilateral period 393 



1. (a) Bilateral division of the posterior primary micromeres ; (b) 



third division of the first somatoblast; {c) fission of the second 

 somatoblast 394 



2. History of the first group of micromeres; origin of the cross and of 



the prototroch 395 



V. Bilateral period of cleavage 399 



1. General histo)y of the upper hemisphere ; origin of the head-kidney . 400 



2. History of the head-kidney . . ._ 402 



361 



