MONOGRAPH OF THE EXISTING CRINOIDS. 469 



extreme rapidity, becoming continually smaller. Only in the first stages can 

 indirect nuclear division be clearly recognized. 



Sections through animals in which the gut is completely filled show that the 

 wall of the gut itself has no definite cell boundaries, and inwardly passes imper- 

 ceptibly into the yolldike interior mass. But along the walls of the gut the nuclei, 

 in the resting stage and mostly very poor in chromatin, are visible. These are very 

 clearly diiferentiated from the elements of the inner mass, which are exceptionally 

 small, strongly stained, and arise from the nuclei of the cells which have passed 

 into the gut lumen, by their relatively large size, the possession of a distinct outer 

 membrane, and the always rounded shape. Apparently in the breaking down of 

 the cells within the gut lumen the chromatin substance within the nuclei breaks 

 in pieces and becomes irregularl}' divided, giving rise to the strongly colored 

 specks abundant within the central mass. In addition to these specks, probably 

 nuclear fragments, there are others, larger, with strongly stained inclusions. 

 These may be looked upon as products of the plasma itself, though this is not 

 certain. Later these disappear also. 



A completely broken-down yolklike mass is not always formed within the gut 

 lumen. In many cases, probably when relatively few entoderm cells have entered, 

 one sees mostly separated cell fragments with relatively little yolklike material. 

 But the ultimate fate of the cell mass, entire resorption through the wall of the 

 gut, is the same in all cases. 



Though the entry of entoderm cells into the gut lumen probably is never 

 entirely stopped, in some cases it is very greatly reduced, and then from the single 

 layered epithelium of the gut there appear protoplasmic pseudopodia-like processes 

 extending inward on which isolated cells lie. 



Later the cell borders again make their appearance and the gut wall appears 

 as an epithelium of cylindrical cells of which the nuclei lie near the outer ends. 



It can now be seen that a part of the elements of the inner mass has been 

 taken up by the walls of the gut, while more is about to follow. The distinctness 

 of the elements filling the lumen from those composing the gut epitheliimi can be 

 determined with greater clearness. The former advance to the outer cell ends of 

 the latter and become gradually absorbed so that finally only chromatin bodies are 

 to be seen, which likewise finally disappear. The nuclei of the gut epithelium are 

 easily differentiated from the elements within the gut lumen since they exhibit all 

 the characteristics of resting nuclei. 



Immediately after the attachment the entire gut appears extended and balloon- 

 like, but very soon the ventral side becomes concave in such a way that the chief 

 mass of the gut lies on tlie originally left side near radii I and II, while a smaller 

 part, appearing as a hornlike process of the other, occupies the right side, its blind 

 end being ventrally on the right between radii IV and V. 



The union with the vestibule takes place through the esophagus on the left 

 side ventrally between radii I and V. 



The difference between these two portions of the gut becomes more and more 

 pronounced. The one lying to the left and dorsally again assumes a more or less 



