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 on the 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 l^y 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 

 tendenc}^ 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 (PI. XXX, figs. 78, 79). 



