26 LILLIE. [Vol. X. 



the case. However, according to Wilson, ^^the residual telo- 

 blasts move apart so as to leave a triangiUar space between them 

 covered with small transparent cells {dor.) and at the same time 

 they gradually recede from the prototrocJi towards the lower pole. 

 This change in position of the teloblasts is of fundamental 

 importance, since I believe the triangular area to represent the 

 middle dorsal region of the adult body, and the residual telo- 

 blasts to mark the posterior limit of the ventral plate." (The 

 italics are Wilson's.) This quotation alone would give one the 

 impression that the middle dorsal region was not formed from 

 the first somatoblast. However, later on, p. 419, Wilson says 

 that "the small cells which separate the posterior teloblasts 

 from the prototroch" are "the descendants of x'^ and (.?) of xf", 

 x^. Fro77t the latter, as I believe, arise the cells that occupy the 

 triangular area between the two residual teloblasts after their 

 divergence. This area afterwards forms the middle dorsal area 

 of the trunk." Thus in Nereis as in Unio the median dorsal 

 as well as the ventral surface is a product of the first somato- 

 blast. There is in Nereis, but not in Unio, a latero-dorsal 

 strip of the trunk epiblast which is derived from a^ and c^. 

 In Nereis the formation of the ventral surface from the first 

 somatoblast is unmistakable, the origin of the dorsal surface is 

 not so apparent. In Unio the reverse is the case ; thus at 

 first sight it seems as though there were a reversal of sur- 

 faces, so that cytogenetically the dorsal surface in Unio 

 would correspond to the ventral surface in Nereis ; but that 

 such is not really the case is sufficiently apparent on a closer 

 analysis. 



I am under obligations to my friend, Mr. A. D. Mead, for 

 permission to record his observations on this subject. Mr. 

 Mead has found in Amphitrite, one of the tubicolous poly- 

 chaeta, that the collective ectoderm of the trunk, dorsal and 

 lateral as well as ventral, is formed from the first somato- 

 blast. " The small transparent cells " (" dorsal cells " of Wilson) 

 form the dorsal ectoderm of the head segment only. Unless 

 Nereis differs radically from Amphitrite, Wilson has been in 

 error in regard not to the origin but to the fate of the 

 dorsal cells. 



