THE LIFK-HISTORY OP NUCULA DELPHINODONTA. 319 



least none conld be found on preserved specimens. They 

 are concerned in the formation of the shell-gland. 



The embryo is still nearly spherical, and so opaque that, 

 while alive, internal changes cannot be followed. A few 

 cells, probably the beginning of the mesoderm, lie above 

 and by the sides of the gut. About this time some of the 

 surface cells around the blastopore divide, and push in to 

 form a stomodeeum. Other cells near the blastopore become 

 enclosed by the surface cells, and together with cells probably 

 derived from those forming the stomodaeum, finally form a 

 portion of the new ectoderm, that soon covers the body of 

 the embryo inside of the test. When the ectodermal layer 

 is complete it joins, but does not enclose, the stomodaeum. 

 In position as well as origin the storaodaeum is ectodermal. 



Before the ectodermal layer is complete the embryo begins 

 to elongate, and the surface cells close in over the shell- 

 gland from the sides and anterior end. At the same time 

 the surface cells become arranged in rather definite rows. 

 It is very difficult to get satisfactory views of these cells in 

 whole mounts, but there seem to be five rows, beside a group 

 at the anterior end that forms the apical plate. Two or three 

 of the posterior rows are interrupted in the region of the 

 shell-g-land, but this interruption disappears as the shell- 

 gland becomes closed in. Closing is never complete. A 

 small opening is left dorsal to the blastopore, separated from 

 it by the width of one test cell (fig\ 15). The anus comes to 

 lie near this opening at a later stage (fig. 24). Before the 

 shell-gland is covered the gut turns towards the dorsal side 

 (fig. 11, my.), and the mesoderm cells take up a position near 

 the posterior end of the embryo. Two of the mesoderm 

 cells are large, and have very large and conspicuous nuclei. 

 These cells are far posterior, and lie side by side. 



Soon after the shell-gland is covered, the gut begins to 

 grow posteriorly, almost, if not quite, in contact with the 

 shell-gland dorsally, and separated from the stomodseum 

 ventrally by a few mesoderm cells (fig. 15). A small space 

 appears among these mesoderm cells that later becomes con- 



