No. 2.] STOLONIZATION IN AUTOLYTUS VARIANS. 297 



Ejnbjyonic Region. 



In this region (PI. XIII, Fig. 6, r.e.) I have included in the 

 description of the external characters of the chain the very 

 youngest segments which have been developed from the region 

 of proliferation, and also such older segments which have already 

 indicated by their general appearance the positions they will 

 occupy in the mature stolon, and in which can already be dis- 

 tinguished the outlines of a stolon (St. A), but all of which I 

 regard as having been derived as outgrowths from the region 

 of proliferation. PI. XIII, Fig. 18, represents a transverse sec- 

 tion through the segment immediately following segment 35, and 

 possesses the characters presented by a very young setigerous 

 segment just after its external outlines, as represented in PI. 

 XIII, Fig. 6, have become distinct. Its structures differ very 

 little from the embryonic structures of the preceding segment, 

 and it presents the very simplest and youngest structures which 

 can form a distinct segment. The mesodermal tissue is quite 

 prominent and fills the entire coelomic cavity. Dorsally and 

 ventrally the tissue appears quite loose, but laterally two dis- 

 tinct denser regions may be distinguished ; the cells of the 

 dorsal region {fn.eJ) being the less compact and forming a 

 larger area, while those of the ventral region {in.e.") are more 

 compactly arranged and are confined to a smaller area near the 

 ventral cord. Between these two regions the tissue presents 

 the looser appearance seen in the dorsal and ventral regions. 

 The dorsal (d.v.) and ventral {v.v.) blood vessels can readily 

 be distinguished from the surrounding tissue, while but a few 

 poorly defined fibers appear to mark the position of the circular 

 and longitudinal muscles. The ectoderm can readily be distin- 

 guished from this mass of mesodermal tissue by the size of the 

 nuclei and the direction and appearance of the cells and muscle 

 fibers. As in the region of proliferation, this tissue is some- 

 what thicker and contains many nuclei placed close together, 

 and indicates by the numerous outlines of cells the rapid forma- 

 tion of new ectodermal tissue. Ventrally the nerve cord {v.c.) 

 passes through this tissue in three distinct strands, all of which 

 are separated from one another and are surrounded by cells 



