362 PROCEEDINGS OF THE ACADEMY OF [April, 
in this respect Thalessema presents an ancestral condition similar to 
that of the Polyclad, though this does not necessarily imply close 
genetic relationship. Moreover a descending series may be formed 
both among Annelids and Mollusks of forms in which the presence of 
ecto-mesoblast gradually merges into conditions in which it has totally 
disappeared, showing that in these groups ectodermal formation of 
mesoderm is onthe decline. The increasing number of cases reported 
in which ecto-mesoblast is larval in fate tend also to support this con- 
clusion, nor do the results of Meyer, showing that much of this building 
material is used for adult structures, offer a serious objection, since it 
is a well-known fact that nature is not prodigal of the living substance 
on which it works, and the secondary application of ancestrally obsolete 
material is a fact of almost universal occurrence. Nor can I see that 
the later origin of ecto-mesoblast necessarily indicates its late phylo- 
genetic appearance, as some have argued, since the early origin of 
ento-mesoblast, if associated with the future elongation of the animal, 
might well be supposed to be directly explained by the precocious 
segregation of this layer in those forms in which its development is 
so intimately connected with future growth and development. The 
early appearance and teloblastic growth of ento-mesoblast in the pos- 
terior region of Annelids and Mollusks has directly led to decrease of 
the radially appearing mesoblast. The Polyclad, which shows no 
endo-mesoblast, has failed to develop such a formation, though a 
tendency in that direction may be appearing, being marked by the 
bilateral division of one of the endodermal derivatives (Wilson). 
The fact that ecto-mesoblast as well as ento-mesoblast has been shown 
among Annelids to arise from the same quadrant (Aricia, Podarke, 
Thalassema) argues, it seems to me, conclusively for an entirely 
separate mode of origin of the two. 
Closure of the Blastopore. 
With the segregation of the secondary mesoblast changes appear in 
the form of the gastrula. Heretofore its shape has been broadly oval, 
the antero-posterior axis being the shortest, but at this period two 
regions of growth become manifest leading to marked change of form. 
The multiplication and growth of cells of the second quartet in the pos- 
terior region increase in activity, ever pushing forward the apical pole 
area, while at the same time the region just anterior to the apical pole 
is seen to be rising from the surrounding surface, forming a pointed 
projection, the summit of which lies at the anterior end of the forward 
arm of the cross (Pl. XXX, figs. 78, 79). 
