76 bulletin: museum of compaeative zoology. 



formed upon either side of the median line, extending forward from the 

 posterior lip of the blastopore, two lateral bands of mesodermal elements. 

 These bands are shown in longitudinal section in Figures 48, 49, and 50 

 (Plate VII.) ; in transverse section, in Figures 45 and 4G. In Figure 48 

 the band consists of five cells, the posterior one of which in this stage 

 is the largest. The band is somewhat curved, so that the anterior ends 

 diverge from the more closely approximated posterior cells. In Figure 

 50 the lateral band contains six cells, the two at the posterior end 

 being in a mitotic state. The position of their spindles is significant of 

 the manner in which the bands have arisen, viz. by proliferation 

 anteriorly from the posterior cell and its products. The " pole cell " 

 as such is not sharply distinguished from the rest of the band by its 

 size, as it is in Umbrella and Nereis. The position of the nuclei of two 

 of the cells is suggestive of a division in a plane coinciding with the 

 axis of the band. I have however never found a spindle in a plane 

 perpendicular to that axis, though spindles parallel with the axis of the 

 band are frequently found. 



I have seen no evidence whatever that any of the cells of these meso- 

 dermal bands are derived from any other source than the primitive 

 mesoblast, d 72 . It is of course impossible to prove that none of the 

 cells can have come from the external layers, either by migration inward, 

 or by the division of a superficial cell in a plane parallel to the surface ; 

 but in the absence of any evidence that this does take place, and with 

 such proof as Figure 50 (Plate VII.) gives, it seems not unreasonable 

 to hold that the entire mesoderm is derived from the one cell, d 72 . A 

 comparison of the origin and development of the mesoderm in Nereis 

 and Limax shows a precisely identical origin in the two forms. In 

 Nereis, however, the mesoderm shares in the generally much more 

 accelerated development, so that, although it appears at about the same 

 cell stage in both forms, the relative number of mesoderm cells in Nereis 

 in the later stages is much greater than in a corresponding stage of 

 Limax. The accelerated division of the mesodermal quadrant (D) in 

 the cleavages of the different quartets, as noted by Wilson and shown 

 in the table of the cleavage of Nereis, may be a manifestation of this 

 same accelerating force. I have not been able to find any trace of such 

 a differentiation in the cleavage of the quadrants in Limax, where there 

 are no early appearing larval organs and little acceleration in the devel- 

 opment of the mesoderm. On the other hand, Lillie ('93) has been 

 able to establish in Unio the same tendency of the quadrant D to 

 precede in division ; but in Unio again there is a very early develop- 



