MONOGRAPH OF THE EXISTING CRINOIDS. 445 



third band. The depth of this invagination varies greatly and is often very 

 considerable. 



The internal organization now shows very important changes. The two 

 coelomic sacks, which in the preceding stage were united by a fine tube, have now 

 become entirely separated from each other. A knob-like swelling on the median 

 wall of the right coelome, which persists for some time, is the last remnant of this 

 connection. 



Simultaneously with the thinning of the walls the two sacs increase in size and 

 become pocket-like, with the inner and outer walls lying very near each other, so 

 that tiie lumina are very narrow. 



The right sack has extended itself dorsally almost around the entire body 

 cavity and has reached over on the dorsal side somewhat to the left; also it has 

 grown considerably anteriorly, where it now covers the gut. 



The left sack has extended itself less far anteriorly, and in its anterior part is 

 much less widened ventrally, so that it lies on the dorsal wall of the hydrocoele, 

 pushing to the right with its dorsal margin and thus forming a sort of mesentery. 

 It has extended especially posteriorly and there surrounds the gut, meeting the 

 posterior border of the right sack. 



Thus the mesentery on the dorsal side runs almost in the median plane, though 

 a little to the left; posteriorly the nearer it approaclies the ventral side the more 

 it shifts to the right. It forms approximately a half circle, which opens upward 

 and ventrally, since on the ventral side the ventral corners of the two ccelomic sacks 

 are at this stage very widely separated from each other (through an angle of about 

 90°), and also anteriorlj' approach the edges only in the dorsal region. 



As from the beginning, the coelomic walls are single layered, but the cells have 

 now become perceptibly smaller and lower. 



While these alterations in tlie ccelomic sacks have been taking place the two 

 sections of the mesentero-hydroccele have become completely separated. 



The primitive hydrocoele is somewhat constricted dorso-ventrally and lies 

 closely on the thickened ectoderm wall, almost entirely on the left side of the 

 embryo, toward which side all the internal organs are more or less displaced. 



Dorsally it extends almost as far as the gut itself, to the posterior end of the 

 left coelome; anteriorly it reaches over the gut and covers it in its central part. 

 This anterior dorsal portion of the primary hydrocoele becomes differentiated as 

 a tubular structure, which runs almost parallel to the ventral surface from right 

 to left, but in the center of the embryo crosses the chief axis not quite at right 

 angles ; this is the parietal canal. 



Histologically the wall of the primary hydrocoele is throughout a single-layered 

 epithelium of cylindrical cells of which the prominent nuclei lie near the inner 

 border. 



During the separation of the parietal canal the histological contrast between this 

 and the secondai*y hydrocoele sack becomes progressively more and more marked; 

 the former is composed always of smaller, at first cubical and later more flattened 

 cells, while in the latter the form of the cells appears not to be essentially altered. 



