6 1 8 WILSON. 



EXPLANATION OF PLATES. 



[Unless otherwise indicated all of the segmentation stages are placed with the plane of first cleav- 

 age vertical to the page. Macromeres are designated throughout by capital letters, micromeres by 

 small letters, cleavage planes by Roman numerals.] 



A, B, C, D. The first four blastomeres and later the primary macromeres. 

 A,^ B,^ C,^ D.^ The secondary macromeres, which correspond in time of origin 

 to the " second group of micromeres " in annelids and mollusks. 

 «,' 1^,' <:,' d} First group of micromeres. 

 «,3 b,^ c,^ d.^ 



'.''.' > Second group of 



«,'•" b,^-^ ^,2-' d.^-^ ) ^ ^ 



} Third group of micromeres. 



a^-d 5 ) 



> Fourth group of micromeres. 

 dr-^-d?-^ ) 



micromeres. 



