PEDAL LACERATION IN ACTINIANS. 99 



the nucleus, it is impossible in this case to determine whether we 

 have to do with fibers of the first or of the second of the two 

 above-mentioned kinds. In other words the fibers in question 

 may be in the process of becoming differentiated from epithelial 

 cells, or they may be arising through the division of previously 

 existing muscle cells of the second type. At this stage there 

 is no indication of interstitial cells or of developing gland cells 

 in the ectoderm. 



In the endodermal layer of the same section there is exhibited 

 a similar condition of the tissue: Cell walls are entirely wanting; 

 the nuclei, however, by their position indicate a potential cellular 

 arrangement of the cytoplasm. A particularly striking feature 

 as regards this layer is brought out by a comparison of the 

 figure under discussion and Fig. 7, PI. II. In the last-mentioned 

 figure the endoderm, throughout most of its extent, is consid- 

 erably thicker than the ectoderm, while in the new tissue the 

 former is generally so thin that the single zooxanthellse are greater 

 in diameter than the layer of cytoplasm making up the endoderm 

 and consequently they cause a prominence extending into the 

 cavity of the tentacle wherever they are situated. In some few 

 instances the zooxanthellae are placed one above the other, and 

 n such cases there is either a generally thickened area of the cyto- 

 plasm or else a cylindrical prominence, as is seen on one side of 

 the figure. All cellular differentiation is entirely lacking in 

 the endoderm. At no place about the circumference of this 

 layer is there any indication of the layer of endodermal (circular) 

 muscles, which are in normal Aiptasia tissues, when seen in trans- 

 verse sections of the animal, much more prominent than the 

 ectodermal muscles. 



At the base of the fragment where the rearrangement of the 

 materials is proceeding at a relatively slow rate there seems never 

 to be a breaking down of the tissues comparable to that just 

 described. The mesogleal layer becomes thinned out progres- 

 sively so that at first it is of its usual thickness on one side of the 

 fragment that side originally opposite the point where the frag- 

 ment was torn off from the parent animal while on the opposite, 

 more actively growing, side it has become relatively thin (Fig. 13, 

 PI. III.). The cellular layers retain practically their original 

 characteristic appearance, although, as is shown in Fig. 15, 



