444 WILSON. [VoL. III. 
blastopore, though owing to an acceleration of development, 
they may be already differentiated in the blastula. After the 
narrowing of the blastopore, this position is found to be anterior 
to the anus (apparently latero-ventral) on either side the median 
line—a position which very nearly corresponds to the seat of 
mesoblast-formation at the hinder ends of the germ-bands in Lopa- 
dorhynchus and similar types. There can be little doubt, there- 
fore, that further study of the matter will show the germ-bands 
of Lopadorhynchus, and its embryological allies, to have nearly the 
same relation to the blastopore as in the other types, for the 
evidence tends to show that the blastopore of all annelids occu- 
pies at first the entire ventral surface. The seat of mesoblast 
formation has, however, shifted slightly, having passed outward 
from the lip of the blastopore, so as to lie, after closure of the 
blastopore, in the ectoblast, and the apparent effect of this change 
has been heightened by a retardation in the formation of the 
mesoblast. Evidence is not wanting that precisely such a shift- 
ing has taken place in other animals, usually, however, in con- 
nection with the formation of a primitive streak, by coalescence 
of the lips of the primitive blastopore. Such a case is that of 
Phoronis (Caldwell, Nos. 11, 12), which is of special value in 
this connection, since the primitive blastopore has the same 
position as in annelids, closes in the same manner, and there 
can be no question of the mesoblastic shifting. The blastopore 
narrows to form the mouth, as in Lumdbricus, but a median ven- 
tral “primitive streak” is left along the line of the closure, 
where the layers remain for some time in fusion. The meso- 
blast arises from three distinct centres of growth situated along 
the primitive streak, but differing in their relation to it. The 
anterior part is formed within the lip of the blastopore, and 
hence from the entoblast, the middle part from the lip itself, 
and hence from the indifferent cells of the primitive streak, the 
posterior part just outside the lip, and hence apparently from 
the ectoblast. There can be little doubt that these three modes 
of mesoblast-formation were originally alike, and were after- 
wards differentiated by slight changes of position with refer- 
ence to the lipof the blastopore. It remains to be seen whether 
a primitive streak can be found in the annelid embryo, but in 
any case there can be little doubt that the apparent contradic- 
tion between the two types of mesoblast-formation in annelid is 
