434 WILSON. [Vol. VI. 



mark in his conclusions. He correctly describes the mesoblast 

 as arising by a proliferation of the ectoblast near the posterior 

 lip of the blastopore. This region of the ectoblast, however, he 

 supposed to be derived from two large superficial cells i^e., 

 Figs. 4 11., 4 «.«., PI. XXIII ; their relations to the cells "^" 

 and " (^ " of the last stage were not determined), which he cor- 

 rectly regarded as homologous to the " primary mesoblasts " of 

 Gotte. His figures leave no doubt that these cells are the two 

 posterior proteloblasts {X, X). His conclusions, therefore, dif- 

 fer from Gotte's mainly in the form of statement, since he 

 regarded the "primary mesoblasts" as forming part of the 

 " ectoderm," while Gotte regarded them as of entoblastic origin. 



In my preliminary paper I first showed the true relations of 

 the somatoblasts to the germ-layers, and v. Wistinghausen's 

 paper (suppressed trochophore of N. Dumerilii), which appeared 

 soon afterwards, contained a welcome confirmation of my gen- 

 eral results. According to this author {whose description of the 

 first three cleavages coincides with Gotte's and my own), three 

 sets of micromeres are separated from the macromeres, and the 

 origin of the somatoblasts is the same as in the American spe- 

 cies. He has, however, overlooked the left posterior micromere 

 of the third group {d^), the place of which, as he supposes, is 

 taken by the second somatoblast. I am persuaded that this is 

 an error, for two reasons ; first, because I also for a long time 

 overlooked this micromere, which is very hard to see on account 

 of its transparency and its position, wedged in as it is beside 

 the somatoblast ; and second, because a precisely corresponding 

 micromere is found in the embryo of the gasteropod Crepidida 

 (as described by Conklin), the cleavage of which agrees with 

 that of Nereis, step by step, up to the close of the spiral 

 cleavage.^ 



Regarding the fate of the somatoblasts, v. Wistinghausen's 

 general conclusions agree closely with my own. They differ, 

 however, in the interpretation of the " secondary mesoblast," 

 which V. Wistinghausen calls the " untere Urzellen des Rump- 

 fes," and which, as he asserts, give rise to the anterior portion 

 of the ventral ectoblasteal. The primary mesoblasts {i.e. after 

 separation of the secondary mesoblast) are called "myoblasts." 

 Since the second somatoblast contains ectoblastic as well as 



1 I have since found this micromere in four other genera of Polychata. 



