IOO LEWIS R. GARY. 



PL III., the ectoderm on the side where the new growth is the 

 most rapid, i. e., where there are no old mesenteries in the figure, 

 is much lower than on the opposite side. 



When in the later development the cellular elements of the 

 ectoderm and endoderm become delimited by the appearance 

 of cell walls it is noticeable that all of the cells show at first an 

 embryonic character in that they are relatively wide in proportion 

 to their height (compare Figs. 10 and n with the old part of the 

 tissue in Fig. 14). In the ectoderm of the column wall in Figs. 10 

 and ii, and more especially in the lining of the stomodeum of 

 this individual, there is a very marked paucity of the nematocysts 

 and gland cells, so that it is apparent that there has not been 

 an increase in these cellular elements to keep pace with the 

 increase in the epithelial cells. In the older embryo from which 

 the series of figures from 15 to 18 was taken the nematocysts and 

 gland cells have reached nearly their normal number and dis- 

 tribution throughout the newly formed tissues. 



In the sections shown in Figs. 10 and n the endoderm cells 

 are in general broader than the ectoderm cells and not generally 

 of greater height. In the older tissue (Fig. 18) there are no 

 apparent cell boundaries in the endoderm and the zooxanthellse 



are much more numerous. 



SUMMARY. 



1. The change in the form of a laceration piece, leading up to 

 the acquisition of the typical actinian shape, takes place through 

 the upgrowth of the tissue about the orifice where it was torn 

 off from the parent. 



2. The permanent mesenteries arise as new growths in the 

 undifferentiated tissues of the oral end of the laceration piece. 



3. The first twelve mesenteries do not appear in the sequence 

 followed by those in egg embryos. 



4. As development goes on the old mesenteries those brought 

 over from the parent when the fragment was torn off become 

 restricted to a proportionately shorter and shorter part of the 

 base of the young actinian until they are finally entirely resorbed. 



5. The mesenteric filaments are formed, just as in egg embryos, 

 from a downgrowth of the ectodermal lining of the stomodeum. 

 Their trilobed condition arises through the differentiation of this 

 tissue. 



