THE LIFE-HISTORY OF NUCULA DKLPHINODONTA. 335 



finally form the test, the apical plate, the cerebral ganglia, the 

 stomodaeum, and a considerable portion of the future ecto- 

 derm of the embryo ; and large ones that form the shell-gland. 



The endodermal pouch is carried further into the interior 

 by the division and pushing in of ectodermal cells in the 

 region of the blastopore (figs. 9 and 11, mg.). In this way a 

 long stomoda3um is formed on the ventral side of the de- 

 veloping embryo. The ectodermal covei"ing of the later 

 embryo, exclusive of that derived from the shell-gland, seems 

 to be formed in connection with the formation of the stomo- 

 daeiim, by cells that wander in from the region of the blasto- 

 pore, and perhaps from cells derived from the stomodaeum 

 itself. 



About the time that the stomodaeum begins to form, a few 

 cells, two of which are quite large and conspicuous, make 

 their appearance by the sides of the endodermal pouch, and 

 extend between it and the shell-gland. These are meso- 

 dermal cells. Their exact origin has not been traced. As 

 the embryo elongates, the two large cells come to lie near 

 the posterior end of the embryo (fig. 19). They probably 

 correspond to similar cells that have frequently been de- 

 scribed for other forms. Similar cells are found in Yoldia in 

 a corresponding position. 



Test. 



As the result of the first few cleavages a number of large 

 cells become covered on one side by a cap of smaller cells 

 (fig. 4). A part of the smaller cells become covered with cilia, 

 about the time that the gut is formed (fig. 8) ; others near the 

 blastopore divide rapidly and form the stomod^ura (figs. 9 

 and 11) ; still others form the cerebral ganglia ; while others 

 in the region of the blastopore wander in and form a part of 

 the future ectoderm. 



The cells that bear cilia are concerned in the formation of 

 the test and apical plate. These cells soon cover the surface 

 that is not occupied by the shell-gland and the cerebral 

 ganglia. Both the apical plate and the cerebral ganglia are 



