202 GEORGE W. TANNREUTHER 
different origin. This same point was previously urged by 
Meyer. 
The condition, however, found by Wilson in Nereis and Lum- 
bricus does not indicate a hard and fast distinction between the 
two kinds of mesoblast. In Nereis, cells from the anterior end 
of the germ bands separate early and pass forward into the seg- 
mentation cavity where they give rise to the larval musculature. 
This corresponds exactly in structure and function with the 
larval mesoblast of Unio (Lillie) and Podarke (Treadwell). 
In Lumbricus the origin of the mesenchyme is similar to that in 
Nereis. These two kinds of larval mesenchyme have also been 
described by Eisig (98) as occurring in the same individual 
(Capitella, a polychaete annelid). 
In Thalassema and Podarke the larval mesenchyme arises 
directly from the ectoderm, while in Nereis and Lumbricus it 
arises from the anterior ends of the mesoblast bands. Accord- 
ing to Treadwell, no one has yet proven that no ‘mesenchyme’ 
arises from the germ bands in cases where a larval mesenchyme 
exists. If we accept Wilson’s view that mesenchyme and meso- 
derm are different phylogenetically, we must regard the two sets 
of larval mesenchyme which have the same structure and fune- 
tion, as non-homologous, or we must regard the mesenchyme 
and mesoderm as morphologically the same tissue and the dif- 
ference in their modes of origin as of no significance. Further- 
more, Wilson has pointed out that the trochophore, as it occurs 
at present, is more than a mere ancestral stage, for it contains 
in a concentrated form the anlage of the whole future body. 
According to Mead, the ectoderm behind the first septum in 
Amphitrite arises from a group of cells which surround the proc- 
todaeum of the young trochophore and are descended from a 
single cell, the ‘first somatoblast.’ The same is true of other 
trochophore forms. There is no need to assume phylogeneti- 
cally a new formation of ectoderm for the body as distinct from 
that of the head. Neither is there any necessity to assume a 
distinct phylogenetic origin of the larval mesoblast from that of 
the mesoderm. 
