190 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Vermes. 
a. Annelida. 
Origin of Mesoblast-Bands in Annelids.* — Mr. E. B. Wilson has 
long been seeking to reconcile the observations of those who, following 
Salensky, have described the mesoblast-bands of Annelids as arising in 
some cases by direct proliferation from the ventral ectoblast, with those 
of such as Kowalevsky, who have demonstrated that in some cases the 
mesoblast first appears in the form of large cells (teloblasts), by the 
proliferations of which the paired mesoblastic bands arise. He hopes 
that the study of the early stages of Nereis limbata and N. megalops 
will help to clear the way. The eggs of these worms are extraordinarily 
favourable for investigation, as they are transparent, of comparatively 
large size, and can be procured in abundance. As in Lopadorhynchus 
and other types, the trochophore seems to consist at first of two layers 
only. The mesoblast, like the neural foundations and those of the seta- 
sacs, arises directly from a thickened bilobed ventral plate ; that is, it 
seems to arise from the ectoblast. But closer examination shows that 
the cells of this ventral plate differ from the remaining cells of the 
outer layer ; they are larger, differently granulated, and, with certain 
reagents, assume a brownish colour that marks them off very sharply. 
It is possible, therefore, to trace their origin, and it may be found that 
the mesoblast is completely segregated in the anterior part of the plate, 
while the posterior part alone gives rise to ectoblastic structures. Each 
of the two divisions of the ventral plate may be traced back to a single 
cell (pro-teloblast) which is obviously homologous to a corresponding 
cell in the early embryo of Clepsine. These two cells the author calls 
X and Y, and he tells us that from the latter arise the mesoblast-bands, 
and from the former the neural plates, the seta-sacs and other structures 
still undetermined. After tracing the fate of each of these cells through 
several stages, Mr. Wilson proceeds to compare the history with that 
of Clepsine and Lopadorhypchus. In Clepsine the large posterior 
macromere first separates off a single micromere (as in Nereis ), and 
then divides into two large cells. The upper right-hand cell (neuro- 
nephroblast of Whitman) has precisely the same relation to the rest of 
the embryo as the first pro-teloblast of Nereis. In Clepsine this cell 
breaks up into eight teloblasts, but in Nereis into four only ; the suc- 
ceeding history of each shows, however, that it is practically certain 
that the first pro-teloblast of Nereis is the homologue of the “ neuro- 
nephroblast ” of Clepsine , while the second pro-teloblast of Nereis is the 
homologue of the common primary mesoblast of Clepsine. These two 
forms agree in all essential points ; and they differ only in secondary 
details — in the ultimate number aud arrangement of the teloblasts, and 
in the temporary position of the products. 
Mr. Wilson thinks that the b’lobed ventral plate of LopadorhyncJius 
must be regarded as the homologue of the ventral plate of Nereis. 
They differ only in the earlier segregation and differentiation of the 
mesoblastic material in Nereis, which leads to the formation of a pair of 
transitory telobasts, which, however, form part of the ventral plate. 
The author then raises the question whether the secondary mesoblasts 
* Journal of Morphology, iv. (1890) pp. 205-19 (6 figs.). 
