EMBRYOLOGY OF BDELLODRILUS 203 
It is evident that in Nereis and Lumbricus, both kinds of mesoblast 
have the same origin, and simply shows a more complete concentration 
of the mesoderm than in Thalassema and Podarke, where the mesen- 
chyme is formed direct from the ectoblast. The mesoblast cells col- 
lected at the posterior end of the trochophore, which are derived from 
M, represent the mesoderm of the body. It is morphologically con- 
tinuous with that of the head, as in Nereis, and is concentrated at this 
point to provide for the elongation as new segments are formed. The 
difference in the concentration of the mesodermal elements, as to 
whether they have a single or double origin in no way interferes, as 
already pointed out, with the morphological unity of the tissue, and 
as to the source of its origin, whether from the ectoderm or from the 
endoderm phylogenetically, we are not able to say (Treadwell). 
Meyer (’01) in his study of the phylogenetic significance of 
the two kinds of mesoblast, gave a view directly opposed to that 
expressed by Treadwell. After an exhaustive review of the whole 
mesodermal question, he concludes that the great mass of evi- 
dence, both embryological and anatomical, points to the con- 
clusion that in annelids, at least, there are two entirely distinct 
forms of mesoblast, the ecto-mesoblast (primary mesoblast) 
and the coelo-mesoblast (secondary mesoblast). Of these two 
he considers the primary mesoblast to be phylogenetically the 
older, and as a rule, to be derived from the ectoderm. The 
coelo-mesoblast, on the other hand, is regarded as a later forma- 
tion, which has originated from the gonad cells. 
The formation of the ecto-mesoblast in annelids and molluscs 
from certain cells of the first, second, third and fourth generation 
of micromeres, can well be regarded as vestiges or survivals of 
the process which occurs in all four cells of the second and third 
quartettes of certain polyclads. The origin of the ecto-meso- 
blast from the ectoderm in annelids and molluscs, partially bridges 
the gap between them and the polyclads. In order to have a 
complete homology of the mesoderm in the polyclads, annelids 
and molluses, it is necessary to find a polyclad in which there 
is a double origin of the mesoderm. The development of the 
polyclad Leptoplana (Wilson) is the nearest representative to 
complete the homology. In Leptoplana only a part of the four 
quadrants of the second quartette contributes to the entire 
mesoderm, the typical condition in polyclads being that all of 
