438 WILSON. * (Vou. III. 
Eupomatus the blastopore, though smaller, has the same position, 
and in both forms it closes from behind forwards, the foremost 
portion persisting as the mouth. The blastopore also has the 
same position in the epibolic types (Clepsine, Euaxrcs, etc.), 
though, as Whitman has shown, its mode of closure has been 
modified through the enormous accumulation of food-yolk in the 
entoblastic part. 
A comparison of Figs. 35 to 44 will show that the mesoblastic 
and neural elements of the germ-bands in Lumdricus may be 
most clearly conceived as forming a longitudinal ring surround- 
ing the region of this primitive blastopore, and that concres- 
cence of the germ-bands takes place throughout the middle 
region of this ring along the line of union of the lips of the 
blastopore, though these relations are somewhat obscured by 
the fact that their various elements do not develop at the same 
rate and the neural ring remains incomplete. Thus the anterior 
part of both the mesoblastic and neural rings is only completed 
after the blastopore has narrowed to form the mouth, and the 
extreme posterior part of the neural ring appears never to de- 
velop. 
This view of the relation of the germ-bands to the blastopore, 
taken in connection with the arguments in favor of the primitive 
character of concrescent growth, points unmistakably to that 
hypothesis as to the ancestral history of the blastopore first sug- 
gested, as far as I am aware, by Biitschli (No. 10), and afterwards 
developed at length by Sedgwick in his well-known paper on the 
origin of metameric segmentation (No. 41). In spite of the 
strong opposition with which this hypothesis has been received 
in some quarters, and in spite of the difficulties which it, in 
common with every other theory, has to encounter, it neverthe- 
less appears to me to be the only one that gives any approach to 
an explanation of the phenomena in question. If we suppose 
the annelid blastopore to have given rise originally to both 
mouth and anus by closure in the middle region (as it still does 
in Peripatus), and that this process represents approximately 
the phylogenetic origin of mouth and anus from the ancestral 
protostome, the explanation of mesoblastic and neural concres- 
cence becomes obvious. The primitive mode of closure of the 
blastopore itself has been modified,—probably through the need 
of a very early establishment of the mouth, — so that the con- 
