18 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



ing of the vestibulum is thinning out in the middle and must be very near the 

 time of opening. The other Pentacrinoids that went so far in their develop- 

 ment did not afford sufficient material for study by means of sections, but 

 as none of them went much farther in their development (in none of them 

 has the formation of the arms begun) there could hardly be expected any 

 noticeable progress in the internal development beyond the stage figured. 



The ectoderm has lost the last trace of its original character, and there is 

 henceforth no distinguishing of the former ectoderm cells from the mesen- 

 chyme cells; also, the glandular cells have completely disappeared. 



The basal disk, which could readily be studied intact in these specimens 

 from the surface film, where no force was needed to loosen the attachment, 

 shows the interesting feature that a rather thick cuticular layer has been 

 secreted. This probably represents the secretion by means of which the 

 attachment is effected. A similar thick cuticula on the base of the stalk was 

 also observed by Seeliger in Antedon (p. 337). The upper side of the basal 

 disk contains numerous small grains which stain strongly in hematoxylin, 

 the cell nuclei among them appearing only very lightly colored (plate vm, 

 figure 1). 



In the vestibulum the thinning out of the high epithelium of its basal wall 

 has just begun (plate vm, figures 1 and 2). The formation of the oral 

 nervous system apparently has not yet begun. 



The hydrocoel ring has just closed (plate vm, figure 6) ; in the anal inter- 

 radius a narrowing of the lumen of the ring is still seen, across which a thin 

 dissepiment goes. This is no trabecule, but the joined end-walls of the two 

 ends of the hydroccel; it is still intact, so that the lumen of the ring is not 

 yet continvious. The narrowing of the hydroccel ring seen in the upper left 

 interradius in plate vm, figure 6, is only apparent and depends on a slight 

 sinuosity of the ring; in the following section it has exactly the same width 

 as in the rest of the circumference, while the narrowing in the anal inter- 

 radius is continuous through all the sections touching the hydroccel. The 

 epithelial lining of the hydrocoel ring has taken on an endothelial character 

 and numerous trabeculae are crossing its lumen. Two pairs of tentacles have 

 developed at the side of each primary tentacle (plate vm, figure 5; plate vn, 

 figure 1). The outer pair of these represents the interradial tentacles. In 

 the sections through the vestibulum the tentacles are seen to be separated 

 from one another by a fine but distinct line (plate vm, figure 4), which 

 probably indicates the existence in the vestibulum of a slimy fluid. 



The communication between the hydroccel and the exterior has at length 

 been established. Plate vm, figure 5, shows the external opening of the 

 hydropore. That it has just been formed is evident from the fact that the 

 hydropore is still closed in the specimen from which plate vm, figure 1, is 

 drawn, and which is otherwise in exactly the same stage; it ends two sections 

 from the one figured, in a small accumulation of cells just below the skin, 



