HABITS, ETC., OP CERKBUATULUS LACTEUS. 183 



compact, and hence not quite as transparent as the rest of 

 the ectoderm. These plate cells divide slowly but steadily, 

 while the surrounding ectoderm cells do not divide at all; 

 hence they are pressed inward as an invagination into the 

 body-cavity. Neither this invagination nor that of the intes- 

 tine is quite symmetrical, the former being inclined toward 

 the anterior end of the pilidium, and the latter toward the 

 posterior end. As they invaginate the plate cells become 

 smaller and more conical or wedge-shaped, the bases pointing 

 inward toward the body-cavity (fig. 70). These changes pro- 

 ceed slowly, so that for two or three days the plate projects 

 but slightly into the central cavity as a rounded swelling 

 (fig. 71). But it grows more and more convex, until it ac- 

 quires a thimble or nipple shape by the sixth day. 



In preserved specimens the apical muscle is usually con- 

 tracted, and the thimble is pulled out into a cone, and by 

 comparing longitudinal with transverse sections it can be seen 

 that the cone is nearly symmetrical in outline (cf. figs. 72 

 and 73). 



A median section shows that the plate is composed of small, 

 crowded conical cells, whose bases show through the ecto- 

 derm as distinct circles (fig. 74). The cells are shorter around 

 the margin and longer toward the centre, each containing a 

 large spindle-shaped nucleus and sometimes a highly refrac- 

 tive nucleolus. 



Both cells and nuclei stain more deeply than the adjacent 

 ectoderm, showing a corresponding difference in their ac- 

 tivity. 



The outer ends of the nuclei are usually pointed, and some- 

 times prolonged into a short, slender, thread-like process 

 (fig. 76) ; but the inner ends are not so prolonged as Salensky 

 has figured for Lineus (45), they are well rounded and plump. 



A muscle fibril connects with the inner end of each of the 

 cells near the centre of the plate, but not with those toward 

 the margin. These fibrils can be seen to enter the plate 

 through the structureless membrane which covers the whole 

 plate, and are lost sight of in the substance of the cytoplasm, 



