362 ■ WILSON. [Vol. VI. 



VI. History of the somatoblasts. Origin of the ventral plate and of the meso- 



blast-bands. Closure of the blastopore. Origin of the stomodaeum. . 404 



1. The first somatoblast 407 



2. The second somatoblast ; origin of the inesoblast 41 1 



3. History of the second and third groups of niicroi7ieres 414 



4. Closure of the blastopore ; the stomodcetiin 417 



VII. General history of the metamorphosis 420 



VIII. Axial relations of the trochophore 426 



IX. Comparison with other forms 432 



1. Comparison with other species of Nereis 432 



2. Comparison with other annelids 436 



3. General comparison with other animals 439 



X. General interpretation of the cleavage . , 443 



The following studies were undertaken, in the first instance, 

 in the hope of clearing up certain perplexing problems involved 

 in the origin of the germ-layers in annelids, especially those relat- 

 ing to the formation of the mesoblast in the polychsetous forms ; 

 and, with this end in view, I sought to find a form in which the 

 detailed history of the mesoblast might be followed in the cleav- 

 age-process, and its precise relation to the other layers thus 

 determined. For a long time the search was fruitless. The 

 eggs of most of the available American forms proved unfavora- 

 ble for my purpose, on account of their opacity, the difficulty of 

 orienting the early stages of development, and the lack of dis- 

 tinctive characters in the external features of the blastomeres. 

 At length, however, my attention was directed to the eggs of two 

 species of Nereis {N. limbata, Ehlers, and N. megalops, Verrill), 

 which soon proved to be of unusual interest and importance, not 

 only for the investigation of the mesoblast-formation, but also for 

 the study of the cleavage of the ovum from a more general point 

 of view. In both forms the mesoblast-bands could be traced 

 back to a single cell and the relation of this cell to the other 

 blastomeres accurately determined. The facts thus established, 

 as I have endeavored to show in a preliminary paper (No. 30), 

 go far to reconcile the various modes of mesoblast-formation in 

 annelids, and, as I believe, throw a new light on the well-known 

 researches of Salensky (No. 21) and Kleinenberg (No. 14) in 

 this field. It soon appeared, moreover, that the early stages of 

 Nereis possessed a more general interest. The ova are extraor- 

 dinarily favorable for a detailed investigation of the history of 

 the individual blastomeres in the cleavage-process. They are 



