14 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



Observations on the beginning of embryonic segmentation in the 

 annehds differ somewhat as to whether the intersegmental divisions 

 appear first in the ectoderm or in the mesoderm, but most students 

 of anneHd development find either that the ectoderm and the meso- 



FiG. 7. — Examples of primary segmentation in polychaete larvae. 



A, Enpamatiis micinatus, trochophore showing primary segmentation of the 

 mesoderm (from Iwanoff, 1928). B, same, later larval stage, horizontal sec- 

 tion showing development of chaetal sacs in primary somites, and extension of 

 posterior part of body (from Iwanoff, 1928). C, Platyncrcis dunwrilii And. 

 & Milne-Edw., nereidogen larva just out of egg, with four primary somites 

 (from Hempelmann, 1911). D, CapitcUa capitafa Fabr., embryo before seg- 

 mentation, ventral view (from Eisig, 1899). E, same, embryo with seven 

 somites and zone of growth formed directly in primary body region (from 

 Eisig, 1899). F, same, later stage with two additional somites formed from 

 zone of growth (from Eisig, 1899). 



E, eye ; I-IX, somites ; Mth, mouth ; Prst, prostomium ; Pyg, pygidium ; SPl, 

 somatic plate; Tl, tentacle; ZG, zone of growth. 



derm are segmented at the same time, or that the first signs of meta- 

 merism are to be seen in the ectoderm. 



In the development of the polychaete Capitella, according to Eisig 

 (1899), on the sixth day after fertilization of the egg, the cells of the 

 ventral somatic plates of the larva (fig. 7 D, SPl) become arranged 

 in transverse rows, and on the sixth day seven or eight somites are 

 already demarked by transverse lines in the ectoderm of the larval 



