PEDAL LACERATION IN ACTINIANS. 89 



part of the fragment and becomes smaller and more rounded, the 

 stomodeum on that side which has come from the growth of the 

 pedal disc comes into contact with the body wall. The first 

 mesentery makes its appearance at the point where the forming 

 stomodeum is in contact with the column wall; although in none 

 of my sections is there evidence of any marked displacement of 

 the endoderm such as would be necessary to bring the two meso- 

 gleal surfaces into contact. As is always true in the formation 

 of a mesentery, the growth of the mesogleal lamella here begins 

 at the extreme oral surface and proceeds downward along the 

 stomodeum and column wall. In laceration embryos the endo- 

 derm is carried down on both sides of the mesogleal lamella so 

 that from its earliest appearance, the mesentery is structurally 

 complete. As the growth leading to the acquisition of the 

 characteristic actinian shape of the laceration piece goes on the 

 stomodeum becomes situated more centrally in the oral disc. 

 While this adjustment is taking place the first mesentery becomes 

 lengthened in the horizontal direction to keep pace with the 

 change in the position of the stomodeum. 



The method of the formation of all those mesenteries formed 

 after the stomodeum has become central in position is the same. 

 The first indication of their appearance is seen as a slight ridge 

 of mesoglea extending aborally from the oral disc at the point 

 where the latter joins the column wall. As the growth of the 

 mesentery continues the mesogleal lamella extends farther from 

 its point of origin, both horizontally along the oral disc and 

 proximally along the column wall. When by its horizontal 

 growth the mesogleal lamella comes in contact with the mesoglea 

 of the stomodeum a rapid downgrowth of the mesentery takes 

 place. Throughout the development of a mesentery the growth 

 is the most rapid where there is contact between the growing 

 mesogleal lamella and that of the body wall, oral disc, column 

 wall or pedal disc. In all such mesenteries the growth of the 

 endoderm keeps pace with that of the mesogleal lamella so that 

 the latter never extends to the free border of the mesentery. 



In Figs. 10 to 14, Pis. II. and III., are shown a number of 

 sections through the same laceration embryo at different levels. 

 In Fig. 10, which passes close to the oral disc, only four complete 



