Vol. 3 J Torrey. — Biological Studies on Corymorplia. 261 



to the establishment of the area of local accelerated growth. 

 When the area of diflPerentiation is established, however, it will 

 only develop an oral extremity, provided it does not adhere to 

 the substratum. 



3. Estahlishment of Body Regions. Elongation of Stem. — 

 After the larva has left the egg case, a somewhat swollen anterior 

 region can be distinguished from a narrow posterior. At the 

 anterior end the enteron is conspicuous (Fig. 9). Throughout 

 the rest of the body, the endoderm is thicker than the ectoderm 

 and is composed of large columnar cells which may almost 

 obliterate the enteron. In twenty-five to thirty-five hours, the 



Fig. 9. Larva, with enlarged oral eud, where enteron is seen. 



larva has assumed the elongated form shown in Fig. 10. Ac- 

 celerations and retardations in the development of the parts 

 relatively to each other in different individuals lead to small 

 variations in the development from this point. 



By the end of the second day, the anterior region is further 

 set off from the posterior by a slight constriction which appears 

 about the middle of the body and fixes the limit between hydranth 

 and stem (Fig. 11). In the ectoderm, the constriction is ex- 

 pressed by a shortening of the cells lying in its course, while, 

 immediately below, the cells are at this stage occasionally length- 

 ened to form a low annular thickening. The coelenteron narrows 

 in the same region, owing partly to the constriction of the body, 

 partly to a local increase in the prominence of the endoderm 

 which serves to define the cavities of hydranth and stem. 



A somewhat older larva is shown in Fig. 12, drawn from life, 

 in two forms assumed within a few minutes of each other. The 

 stem has lengthened. The hydranth is still completely invested 



