No. 2.] STOLONIZATION IN AUTOLYTUS VARIANS. 303 



of no significance. In a similar section of a stolon, a little 

 older than stolon I, another group of nuclei has made its 

 appearance in the cerebral tissues directly over the dorsal 

 vessel, while the lateral groups have become somewhat denser. 

 PI, XIV, Fig. 23, presents a longitudinal median section 

 through the head of stolon 2. At this stage the cerebral 

 thickening is very much greater than in the preceding stolon, 

 the outline of the head has become quite distinct, and its divi- 

 sion from the tissues of the anal segment is quite well marked. 

 The median tentacle {d.m.) appears quite prominent, and its 

 internal structures are not unlike the surrounding structures 

 of the head proper. The cerebral tissues have, however, at 

 this stage already begun to be differentiated into a small cen- 

 tral medullary region (c.b.) and the peripheral cortical area {cm.) 

 of the mature head. The medullary region has appeared 

 directly over the mesodermal tissue in the region occupied by 

 the group of nuclei, which I have described as appearing in 

 a line with the median tentacle in a transverse section of a 

 stolon a little older than stolon I, and owes its origin to changes 

 which have taken place within this group of nerve cells. Ex- 

 amined under a high power, this young medullary tissue pre- 

 sents an appearance similar to the tissue along the border of 

 the medullary substance in the head of the mature stolon, and 

 does not as yet possess the dense fibrillated arrangement com- 

 mon to the medullary substance of this region in more mature 

 individuals. The cortical substance in this plane is composed 

 of cells evenly distributed throughout the region of the head 

 and showing no differences in the median tentacle. The 

 arrangement of the nuclei for the formation of a distinct epi- 

 dermis over the anterior part of the head is not as evident as 

 it was in the transverse section of the preceding stolon ; but in 

 the median tentacle not only have the nuclei an arrangement 

 suggestive of epidermal tissue, but the outlines of the cells are 

 also more plainly visible, and form a marked contrast with the 

 underlying cells. Posterior to the median tentacle the ecto- 

 dermal cells still retain their embryonic spindle characters, and 

 this region is already beginning to contribute toward the for- 

 mation of the buccal segment. 



