EMBRYOLOGY OF BDELLODRILUS 205 
d‘ does not contribute to the coelo-mesoblast. Here then we 
have quite a unique series ranging from those forms where the 
entire mesoderm is ectodermal in origin, or where it is both 
ectodermal and entodermal, to those where it is entirely ento- 
dermal. From the above it is evident that the entire meso- 
blast of polyclads is derived from the ectomeres, and, if homol- 
ogies be any significance, it would be fair to conclude that this 
mesoblast is represented by the ecto-mesoblast in the annelids 
and the molluscs. 
The origin and development of the mesoblast in Bdellodrilus 
contributes but little to the phylogenetic significance cf the 
primary and secondary mesoblast. Here, beyond question, 
when considered from the standpoint of their origin, they are 
one and the same tissue. Both are formed directly from the 
primary mesoblasts. The secondary mesoblast cells are budded 
off from the two primary mesoblasts before the germ bands begin 
their development. Similar conditions are found in other forms, 
as in Lumbricus; here, however, the secondary mesoblast is 
formed later directly from the anterior ends of the mesoblastic 
germ bands. The difference is only in the point of time in their 
formation. In Bdellodrilus there can be no hard and fast dis- 
tinction made between the two kinds of mesoblast. Both 
must be considered as the same tissue phylogenetically. 
4, VARIATIONS IN THE SOURCE OF THE ENTODERM 
In general, as stated above, the ectoderm originates from the 
three generations of ectomeres, the mesoderm from d‘, and the 
entoderm from the remaining cells. The origin of the three 
germ layers, however, depart somewhat from the above rule in 
some of the annelids and molluscs. In some species cells from 
the first, second and third quartettes contribute to the meso- 
derm; in others d‘ gives rise to entoderm as well as mesoderm. 
Tn all annelids and molluses, A, B and C, after the formation 
of the first three sets of ectomeres, are distinctly entodermal. 
The macromere D, after the formation of d*, is likewise entoder- 
mal. In some forms D is the same size as its fellows, in others 
