260 W. R. Coe — Development of the 



the esophagus is comparatively much smaller and its epithelium is 

 correspondingly thicker. 



The cilia on the esophagal cells are for the most part short and 

 scattered. Near the intestine the cells become much thicker and the 

 cilia longer and more numerous. 



The cells making up the blind intestinal sack are always more or 

 less columnar in form, and all are provided with very long and strong 

 cilia. By means of these any food-particles which may pass into 

 the intestine are churned rapidly round and round. In 31. ccBca 

 and C. leidyi the intestinal cells are more highly columnar in the 

 earlier stages (Fig. 2, PI. xxxiii) than in the later (Fig. 3, PI. xxxiv). 

 In C. marginatus (Fig. 6, PI. xxxiii) and in C. lacteus the cells are 

 much more elongated, and their oval nuclei comparatively smaller 

 than in the other two species. 



In that portion of the cell which lies nearest the lumen of the 

 intestine are often found numerous small granules, and sometimes, 

 vacuoles of secretion (Fig. 1, PI. xxxv). This secretion is doubtless 

 a special digestive fluid which is poured out into the canal when food 

 is present. In C. marginatus such granules or vacuoles often appear 

 even before the pilidium is fully formed. 



In many specimens we find that a few of the intestinal cells stain 

 quite differently than most of the others. Two of these cells are 

 shown in Fig. 6, PI. xxxiii. Their protoplasm is more granular and 

 is also vacuolated. They have likewise a much greater affinity for 

 stains. Both Salensky (28) and Burger (5) describe such cells. 

 The former suggests that they may be specialized cells of the nerv- 

 ous system, while Burger considers them to be differentiated 

 gland-cells. There is little doubt that the latter view is correct, 

 especially as the contents of the cells often resemble very closely 

 those of some of the gland-cells in the intestine of the adult. 



The Apical Plate. 



In the course of gastrulation, when the cells of the upper surface 

 of the body begin to flatten out, it is observed that a small cluster 

 of cells at the very apex do not take part in the flattening process, 

 but retain their columnar shape. These furnish the " Anlage " of 

 the future apical plate. The cells divide longitudinally and thereby 

 become much smaller than the neighboring cells, although they 

 remain as long or longer than at first. They thus become highly 

 columnar, and are conspicuous in sections, because of their less gran- 

 ular protoplasm, which shows a special affinity for stains (Fig. 4, PI. 



