No. 2.] STOLONIZATION IN AUTOLYTUS VARIANS. 305 



cavity {coe.) on the first setigerous segment, and has been 

 formed by the upward growth of the mesoderm for the forma- 

 tion of the tentacular muscles, a part of which tissue (c.me.) 

 may be seen on either side of the dorsal blood vessel in this 

 section. 



The next important stage in the development of the head 

 is represented in a longitudinal median section of stolon 4 

 (PI. XIV, Fig, 26). At this stage the structures of the head, 

 as has been seen in the dorsal view of this stolon (PI. XIII, 

 Fig. 6), have become quite well developed, and the rounded 

 mass of cerebral tissue presses down into the underlying tissue, 

 so as to appear partly imbedded within it. At the same time 

 the line of junction of the cerebral tissue and ectoderm of the 

 anal segment anterior to it, which, as seen in PI. XIV, Fig. 23, 

 appeared as a deep groove, has by the increase in the size of 

 the head been completely covered, and is now placed almost 

 ventral to the middle region of the head. The head itself in 

 this section appears quite distinct from the surrounding tissues, 

 and protrudes considerably beyond the lower limits of the sto- 

 lon, the tissues directly underlying it being those belonging to 

 the region of separation. The buccal segment {b.sg.) has also 

 appeared, and forms a distinct segment separated posteriorly 

 from the first setigerous segment (Sg. i) by a dissepiment, 

 while anteriorly it is indistinctly separated from the structures 

 of the head and anal segment by the mass of undifferentiated 

 mesoderm occupying this region. The cortical substance of 

 the head is clearly defined from the surrounding structures, 

 and the brain cells are somewhat more densely crowded than 

 in the preceding sections, while the arrangement of the periph- 

 eral cells suggestive of the presence of an epidermis has disap- 

 peared, and a distinct epidermis in the head structures of this 

 stage of development is visible only in the tentacles. The 

 medullary substance {c.b.) has become more prominent, and 

 dorsally has sent up two processes which at the base of the 

 median tentacle unite and form the nerve («./.) of the tentacle. 

 Similar processes, as may be seen in more lateral sections of 

 this stolon, are also sent out to innervate the lateral tentacles 

 and the eyes. 



