No. 2.] STOLONIZATION IN AUTOLYTUS VARIANS. 295 



In addition to the appearance of this mesodermal tissue the 

 epidermis, particularly in the dorsal region, is somewhat denser, 

 and the cells seem more crowded than in sections of preceding 

 segments. Ventrally the epidermis in this section is not unlike 

 that of preceding segments, the lateral outline of its cells being 

 quite distinct, while in the region of the ventral cord the cells 

 are so closely associated with the ganglion cells {c.n) of this 

 region that the two structures cannot be differentiated. 



PI. XIII, Fig. 16, represents a section through the posterior 

 part of segment 35, PI. XIII, Fig. 6, in a plane just anterior to 

 the first segment of the embryonic region. In this section the 

 mesodermal tissue {m.e) so nearly fills the coelomic cavity as 

 to leave but very small dorsal and ventral spaces (toe}j. The 

 epidermis (ect) has also increased considerably in density, both 

 laterally and dorsally, and the cells are no longer arranged in a 

 simple layer, but are so compactly placed as to give the appear- 

 ance in transverse section of several layers. (In this section, 

 owing to the strong contractions produced by the fixing fluid, 

 the thickness of the epidermis is somewhat exaggerated, par- 

 ticularly dorsally and on the one side.) Compared with the 

 epidermal cells in any other segment of the parent stock, the 

 cells of this region appear to be smaller, less regular, and 

 present more fully the characters of newly dividing cells. 



The alimentary canal {ent^j of this section also presents 

 characteristics different from that in preceding segments. Its 

 calibre is very much smaller, the constriction being already 

 apparent in PI. XIII, Fig, 15, and the decrease in the size of 

 the cells is quite well marked. The abundance of nuclei in 

 the cells would also demonstrate a rapid formation of new 

 cells in this region. 



The general appearance of the tissues of these sections 

 would, therefore, indicate that the posterior part of this seg- 

 ment has undergone a complete change from that of an 

 ordinary segment of the parent stock, and that, while its 

 external appearance remains unchanged, its internal structure 

 has become so modified as to convert its posterior regions into 

 a distinct embryonic center, in which new tissue is being rapidly 

 formed and pushed back to furnish the embryonic tissue, from 



