76 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
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 VIL.) ; in transverse section, in Figures 45 and 46. 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 
perpendieular 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 primitivo 
mesoblast, d'?, 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 coll 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 VIL) gives, it seems not unreasonable 
to hold that the entire mesoderm is derived from the one cell, d?^?, 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 mosoderm 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 Nercis 
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 Noreis, 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- 
