1890.] ; Annelid Descent. I 15 5 
We still have the consideration of the question as to the origi- 
nal significance of the embryonic mesenchym. Here again, as 
it seems to me, the development of the Annelids will help us into 
the right track,—especially that method of formation of the 
primary mesoderm which is found in Lopadorhynchus and many 
other Annelids. The paired rudiment on both sides o the anus 
in the ectoderm represents, according to Kleinenberg, the chief 
neuro-muscle origin for the ventral band and the permanent 
mesodermal structures, but contains, as I think, two different, 
though closely compressed, formative centres,—that of the per- 
manent trunk nervous system, and that of the secondary meso- 
derm. There are here, in addition, a series of regions in the 
ectoderm (considered as “ neuro-muscle Anlagen” by Kleinen- 
berg) that, in my opinion, furnish the mesenchym, represented in 
this special case, to be sure, only by mesenchym muscles. In in- 
vestigating Lopadorhynchus larve I found, however, more of 
such mesenchym “ anlagen” than my predecessor, and as a rule 
lying in the regions where the elements arising from them subse- 
quently are attached as muscles to the ectoderm. 
This discovery, and the circumstance that in other forms there 
arise from the mesenchym, in addition to the muscles, connective 
tissue, the larval and parts of the definitive excretory organs, as 
well as the migrating cells of the primary body-cavity (primary 
leucocytes), and probably also the true blood corpuscles, that 
such migratory cells (as in-the Echinoderms) may be formed also 
from the entoderm, suggests the conclusion that morphologically 
the embryonic mesenchym is not a uniform structure, but repre- 
sents rather the sum of the undifferentiated “ Anlagen” of very 
various organs and tissues, which originally arose quite indepen- 
dently from the ectoderm or entoderm, and wherever necessary. 
It is not as easy to explain the origin of mesenchym structures 
by the migration in many cases of cells from the embryonic 
ceelomic epithelium. In such cases we might assume that the 
various constituents of the mesoderm had united in a common 
origin. Then in cases where the entire mesoderm is formed by 
evagination, or through outgrowths of one or both germ layers, 
the mesenchymatous and ccelomic embryonic elements may be 
