1904.] NATURAL SCIENCES OF PHILADELPHIA. 361 
Here the definitive mesoblast is said to arise from 3¢e and 3d, which 
would be in correspondence with Wilson’s “ecto-mesoblast,” while what 
Eisig considers “larval” or ‘secondary’? mesoblast comes from that 
portion of 4d which Wilson and Treadwell found in Nereis and Podarke 
to form part of the wall of the enteron. These results have, it seems 
justly, been called in question, though the careful investigation from 
which they spring certainly gives credence to their accuracy. ‘Tread- 
well (1901) has called attention to certain figures (Pl. XX XIX, fig. 42, 
to Pl. XL, fig. 49) of Hatschek on Ewpomatus, which show “scattered 
muscle cells in the upper hemisphere of the larva, which could hardly 
have come from the feebly developed mesoderm bands at the posterior 
end of the body”’, and suggests that they are of secondary origin; and 
he likewise calls attention to the figures of Drasche (1884) for Pomato- 
ceros which show similar conditions, though neither of these investiga- 
tors appears to have realized their significance. In a preliminary paper 
on the development of the mesoblast in Thalassema, Torrey (1902) 
derives ecto-mesoblast from all three quartets. ‘‘In all there are 
at least twenty primary cells of this character, but of them only ten, 
arising from the first and third quartets, develop into functional mesen- 
chyme, while at least ten degenerate and are finally absorbed by the 
entoblasts.”” The greater part of the functional ecto-mesoblast comes 
from three cells of the third quartet (3a, 3e and 3d) which correspond 
closely to those which produce secondary mesoblast in Podarke. All 
of the cells arising from the second quartet and which sink into the 
segmentation cavity are rudimentary and in the end entirely degen- 
erate, thus recalling Wilson’s similar conclusions regarding the ‘‘rudi- 
mentary” cells of the definitive mesoblast of Aricia and Spio. At 
least six derivatives of the seven ecto-mesoblast cells which Torrey 
derives from the first quartet have a similar fate. 
The mesoderm of Platodes, Annelids and Mollusks has of late years 
been subject to much study, and various theories have been propounded 
regarding the significance of the manner of formation of the middle 
germ layer of these groups. Without entering into a prolonged dis- 
cussion with regard to this question, afew of the more general points 
may be mentioned. The results above tabulated and my own observa- 
tions lead to the conclusion—which is, of course, not here stated as new 
—that the primitive mesoderm of these groups is represented by that 
which arises from the ectoderm, and which is alone found in the Poly- 
clad (Wilson). The suggestion of Wilson that upon this hypothesis 
ecto-mesoblast might well be found arising from all three quartets of 
ectomeres has just been verified by the work of Torrey, and shows that 
