302 MENSCH. [Vol. XVI. 



Posterior to this region, in the second segment of the stolon, 

 the ectodermal cells are less spindle-shaped and are arranged in 

 a single row, while more posteriorly they present the characters 

 of ordinary epidermal tissue. Anterior to this thickening, how- 

 ever, the dorsal ectoderm comprising the anal segment of stolon 

 A, as we have seen in the study of the embryonic region, is 

 also thickened and presents similarly arranged spindle cells. 

 Since these cells appear in regions in which either nerve cells 

 or simple epidermal cells will be differentiated, they cannot be 

 regarded as being of a purely nervous character, even though 

 they attain their greatest dimensions in positions which later 

 on will be occupied by nerve cells. On the other hand, they 

 may rather be regarded as the typical embryonic cells of this 

 ectodermal tissue in general, the outlines of which are more 

 easily distinguished in regions where the ectoderm is thickest. 

 The appearance of the tissue beneath the ventral cord in the 

 embryonic region would also support this view. 



PI. XIV, Fig. 22, presents the appearance of the developing 

 head of stolon I in transverse section, in the plane of F, PI. 

 XIII, Fig. 17. Dorsally the ectoderm is considerably thickened 

 and presents two lateral prominences which mark the lateral head 

 regions, while between these is a small ectodermal projection 

 which indicates the origin of the median tentacle {d.m.). Lat- 

 erally the ectoderm is also somewhat thicker, though this 

 becomes less marked toward the ventral surface. The out- 

 lines of the spindle cells, as in all other transverse sections, are 

 quite indistinct ; but in the lateral prominences, particularly 

 near the surface, the nuclei of these cells (c.n.) are very much 

 more numerous than in the region of the median tentacle and 

 the dorsal mesoderm. These cell groups indicate the appear- 

 ance of the first pair of eyes. Ventrally the ganglion cells {c.71.) 

 present a grouping very similar to the groupings in the dorsal 

 lateral prominences, and the cells present a very similar appear- 

 ance at this stage. Along the dorsal borders the regular arrange- 

 ment of nuclei in this section would suggest the presence of a 

 distinct epidermis, as Malaquin has been able to distinguish in 

 the mature heads of certain other species of Autolytus ; but 

 as this disappears in later developments, it would appear to be 



