No. 2.] STOLONIZATION IN AUTOLYTUS VARIANS. 309 



Elongation of the Stolon. 



After the outlines of the young stolon (A, PI. XIII, Fig. 6), 

 formed entirely of tissues derived from the zone of proliferation, 

 have appeared, it has been seen in the external description of 

 the phenomena of stolonization that a new region of growth 

 makes its appearance ; and that it is this region, which I have 

 designated as the region of elongation, that contributes to the 

 increase in the number of setigerous segments of the stolon. 

 The external appearance of the segments of the embryonic 

 region in its different stages of development up to a stage 

 of differentiation equal to that of stolon A indicates, according 

 to the description I have given, the absence of such a mode of 

 segment formation. This fact is also supported by the appear- 

 ance of the tissues of these segments in a longitudinal section 

 (PI. XIII, Fig. 17), where in the ventral ectoderm the embry- 

 onic cells {e.e.p.) in every section I have examined are confined 

 to a single segment, which, as has been seen, forms the anal 

 segment of the forthcoming stolon. In a longitudinal section 

 through the last two segments of the stolon represented in 

 PI. XIII, Fig. 7, in which a new segment formed by this region 

 first occurs, the ectodermal and mesodermal structures of both 

 segments are exactly like the structures of the anal segment of 

 stolon A, PI. XIII, Fig. 6, the two segments being distinguish- 

 able from one another only by their external constriction, and 

 hence composed of embryonic tissue. A little later, however, 

 the tissue of the newly formed segment (x) undergoes a higher 

 differentiation, the spindle cells disappear, a coelomic cavity 

 appears in the dense mass of mesoderm, and the segment pre- 

 sents in a longitudinal section an appearance very similar to 

 that of the segment (Sg. i) in the embryonic region of PI. XIII, 

 Fig. 17. The anal segment, in the mean time, retains its embry- 

 onic characters, begins to elongate, and before long adds another 

 segment to the series. Different stages of this process are 

 represented in the longitudinal sections in PI. XIII, Fig. 17, 

 PI. XIV, Figs. 22 and 25, and sufficiently explain the descriptions 

 of PI. XIII, Fig, 7, to make additional figures useless. In all these 

 sections it will be seen that the anal segment is the only seg- 



