No. 2.] STOLONIZATION IN AUTOLYTUS VARIANS. 30 1 



and are arranged in a single layer, but posteriorly, and over the 

 third segment, the tissue is again thickened and the cells again 

 present a spindle-shaped appearance. Ventrally the ectodermal 

 cells of the first and second segments present very much the 

 appearance of this tissue in the mature stolon, but in the third 

 segment the spindle cells ie.e.p) again appear, and this tissue 

 assumes an appearance similar to that in the zone of prolif- 

 eration. The mesoderm presents dorsally and, less distinctly, 

 ventrally a coelomic cavity for segments i and 2, but in seg- 

 ment 3 the cavity is wanting. 



In these three segments, which have in PL XIII, Figs. 6 and 

 17, been designated as stolon A, the external appearances sug- 

 gestive of a distinct stolon have been confirmed by the inter- 

 nal structures of these segments ; and, these segments being 

 typical of the very earliest stolon differentiation, there can, 

 therefore, be recognized in their structures : (i) a thickened 

 ectoderm from which the head will later on be differentiated ; 

 (2) an anal segment of purely embryonic tissue which, as will 

 be seen later on, contributes to the elongation of, (3), the indif- 

 ferent region in which the ordinary setigerous segment is being 

 formed. All of these segments are, as may be seen in the 

 study of longitudinal sections of the different stages in the 

 development of these segments, derived from the region of pro- 

 liferation, so that all the tissue of that region which I have 

 designated as the embryonic region owes its origin to the single 

 center of growth within the last segment of the parent stock. 



Development of the Head ajid Buccal Seg^nent. 



The first differentiation toward the development of the head 

 makes its appearance in the thickened dorsal ectoderm of stolon 

 A (PI. XIII, Fig. 17, c), already referred to in the preceding de- 

 scription. In stolon I of the same figure this dorsal thickening 

 if) has become more pronounced, and in its structure may be seen, 

 for the first time, the fully developed spindle cells as they appear 

 in the early stages of this dorsal thickening. Here the cells 

 have become quite long, and are compactly arranged to form a 

 somewhat rounded dorsal elevation of the ectodermal tissue. 



