14 



b'.^cumes peai'-shaped, with the pole cells at the broad end. This 

 end divides into two parts, so that the egg is now made up of 

 one large mass and two smaller ones, with the pole cells between 

 them (Plate II, Fig. 2). 



The later development of the embryo shows that even at this 

 early stage the egg is not perfectly homogeneous, but that the 

 material which is to give rise to different organs of the body 

 has already been separated from that which is to form others. 



The single cell at the smaller end of the egg is to give rise 

 to the digestive organs of the adult, while the two smaller 

 cells are to give rise to the outer wall of the emhryonic body 

 and all the organs which are derived from it, as the gills, palps 

 and the mantle, as well as the shell, which is secreted by the 

 mantle. The two upper cells divide up into smaller and smaller 

 spherules (Plate II, Figs. 3, 4, 5, 6), until there is a layer of 

 small cells wrapped around the greater part of the surface of 

 the single, large, lower cell. This soon divides into a layer of 

 cells, so that the embryo is made up of two 

 layers of cells : an upper layer of small cells, 

 'cells, and a lower layer of larger cells. The upper, small cells 

 have come from the division of the two spherules occupying 

 the broad end of the egg in the pear-shiped stage, and are to 

 form the outer wall of the body. The lower, larger, cells have 

 come from the division of the single lower spherule of the three- 

 celled stage, and are to form the digestive tract and its derivi- 

 tives. 



The layer of large cells is seen in the section (Plate II, 

 Fig. 7) to be pushed in a little toward the upper layer so that 

 the lower surface of the embryo is slightly concave. This con- 

 cavity is the beginning of the rudimentary digestive tract. 



Soon after the last mentioned stage has been reached, the 

 embryo undergoes a marked change in appearance. A tuft of 

 long hairs, or cilia, makes its appearance at one end, and 

 these cilia begin to lasTi back and forth, thus constituting a 

 sAvimming organ by means of which the little animal forces 

 its way to the surface of the water, where it moves about actively 

 for some time. 



The concavity which marked the beginning of the digestive 

 tract has become deeper, until the edges have almost met around 



