28 LILLIE. [Vol. X. 



nearly with what we know of mesoderm formation elsewhere ; 

 whereas Stauffacher's account stands alone. 



Wilson describes the cells which have been budded off at 

 the surface by the primary mesoblast as forming a pigment 

 area, and later wandering within the segmentation cavity as 

 "secondary mesoblast." The significance of this fact in the 

 interpretation of mesoblast cells of "ectodermal origin" can 

 hardly be overestimated. I believe that the same ultimate 

 fate awaits the two superficial mesoblast cells in Unio. In 

 Fig. G'j it will be seen that between the most posterior cell 

 of the primitive intestine and the mesoderm teloblast there 

 lies a small cell, which is at the most anterior end of the 

 ventral plate, the exact position of the cells m, m. It may be 

 that they are actually these cells, but it is impossible to prove 

 it. In their present position it would be easy for them to be 

 pushed within the segmentation cavity. 



After the teloblasts of the mesoderm have entered the seg- 

 mentation cavity, each buds forth anteriorly a small cell (PI. V, 

 Figs. 63, 66, and 6']). This is the beginning of a mesoblastic 

 germ band on each side. The two bands are parallel, and 

 grow forward in contact in the median line just beneath the 

 large cells of the shell-gland. The teloblasts are forced within 

 the segmentation cavity by the forward growth of the tongue 

 of cells derived from the first somatoblast. The progress of 

 their inclusion can be traced through a series of figures (45, 

 46, etc). X'', X*, and x^-x^ are very active factors in the 

 process. 



(e) Gastrulation and Shell-Gland. 



The archenteron is derived from the entomeres A, B, C, and 

 D. Before they invaginate to form the primitive intestine 

 they increase considerably in number (PL V, Fig. 60). Even 

 before this stage is reached a slight indentation is noticeable 

 in the region of the entomeres. The invagination deepens, 

 and soon forms a small sac communicating freely with the 

 exterior (Figs. 65, 66, and 67). Fig. 65 is a view of this stage 

 from the ventral surface (the entoderm is colored in sepia). 

 Fig. 64 is a view from the dorsal surface, and Fig. 72, from 



