MONOGRAPH OF THE EXISTING CRINOIDS. 427 



one, beneath which the kimen can be discerned. Begeneration proceeds through 

 cell proliferation, cell division being always mitotic, though on account of the 

 small size of the elements involved this is not always easy to determine. Eeichen- 

 sperger is of the opinion that cell division ensues periodically, since it occurs not 

 only in the youngest portions of the regenerated part, but often may be observed 

 in addition far below it, sometimes in the form of a belt or girdle about the water 

 tube. 



Simultaneously with the cell proliferation of the water tube begins the regener- 

 ation of the ectoderm and of the mesenchyme. In the young bud the various 

 elements can not be differentiated, all consisting of rounded or angular cells of 

 different forms which sometimes show small processes. These cells multiply and 

 fill the cavity under the protecting cap, and gradually three layers differentiate 

 themselves, ectoderm, mesoderm, in which the peripheral layer is composed of 

 somewhat cubical cells while the inner mass is made up of more characteristic 

 cell types, and the epithelium of the water tube. The coelomic epithelium also 

 appears as a layer of flat cells. Among the other cells may be found misshapen 

 wandering cells with granular contents, but there are no true phagocytes or lymph 

 cells. 



The rebuilding of the various structures thus results from a proliferation from 

 the parts remaining in the stump, the necessary material being supplied by the 

 wandering cells. 



In the further development of the regenerating bnd there gradually appears 

 a partial separation of the heretofore single, somewhat crescentic, ccelomic cavity. 

 This turns dorsally and divides into two sections, a broader ventral and a more 

 rounded dorsal. At first both parts are in open communication, but very soon 

 individual cells on opposite sides of the ccelomic endothelium protrude and, form- 

 ing connecting processes, divide the two sections of the ccelome more definitely. A 

 little later there is a similar outgrowth of connective tissue from that portion of 

 the ccelomic endothelium which lies on the water tube, which also at first consists 

 only of the thread-like processes of the cells. When the processes of the cells reach 

 the transver.se septiun the ventral canal is divided into a right and left half, as in 

 the case of normal arms ; but communication between the dorsal and the two ventral 

 canals is always maintained by broad lacunae. 



The first appearance of an evagination which will eventually result in the 

 formation of a tentacle or of a pinnule can always be perceived from the accelerated 

 cell division in the region of the water vessel. 



Dorsal and ventral neiTous systems. — Thanks to the pi'esence of a cover- 

 ing of wandering cells the degeneration which takes place in the dorsal nerve 

 cord is usually extremely slight, and immediately after the injury the growth of 

 new fibers commences from the stump. These new fibers are chiefly laid down by 

 new ganglion cells formed from the endotheliiun of the dorsal canal, in which some 

 of the cells become distinctly larger than the others, separate themselves off more 

 or less, sometimes forming a distinct layer, and gradually become differentiated 

 into typical ganglion cells, with processes and vesicular nuclei. 



