No. 2.] STOLONIZATION IN AUTOLYTUS VARIANS. 299 



smaller and more numerous. Ventrally there is even a more 

 marked difference between the cells of the two segments. 

 In the anterior segment (Sg. i) the outlines of the cells are 

 quite indistinct, and in this section of the ventral ectoderm 

 appear very little different from those of a segment of the 

 parent stock. In the second segment, on the other hand, this 

 ventral ectoderm presents an appearance entirely similar to the 

 ventral ectoderm of segment 35 in the plane of B (e.c)\ the 

 outlines of the spindle-shaped cells {e.e.p.) are quite distinct, 

 and this region appears as distinctly embryonic as does the 

 ventral ectoderm of the posterior part of segment 35. 



These two segments accordingly present a stage in which 

 the three embryonic layers have undergone very little change 

 toward the formation of any structure of the mature segment. 

 In the ectoderm the ventral cord forms the only distinctly 

 differentiated structure. The cells beneath it have, in the 

 anterior segment, assumed the appearance of the nerve cells 

 common in this region ; while in the second segment, as seen 

 in longitudinal section, the cells have not undergone this differ- 

 entiation, but still possess the characters of the purely embry- 

 onic cells of the zone of proliferation. The mesoderm of these 

 segments, with the exception of the dorsal and ventral blood 

 vessels and the faint bands of the muscle tissue, presents no 

 differentiated structures, but differs in the two segments in the 

 amount of tissue present. Both segments being embryonic in 

 structure and still possessing characters which make them 

 appear different from one another, if these differences are not 

 due to the differences in age of the segments, we have presented 

 in these two segments two distinct phases of embryonic seg- 

 ment formation. If these differences were due to the variance 

 in the ages of the segments, the second segment, being the 

 elder, would present the greater differentiation ; while, as is 

 quite evident in the longitudinal section of these regions, the 

 anterior segment in reality presents the more differentiated 

 structures. There must be present, therefore, already at this 

 early stage, two distinct types of embryonic segments, each 

 of which is destined by its internal formation to occupy a dis- 

 tinct position in the mature stolon. As will be seen later, 



