3o6 MENSCH. [Vol. XVI. 



The ectoderm on the dorsal surface of the buccal segment, 

 as is also the case in the anterior part of segment i, is still 

 thickened and somewhat embryonic in character ; while in the 

 anterior ventral region the faint outline of spindle cells may 

 be seen immediately posterior to the degenerating tissues of 

 the region of separation (r.i-.). A short time after the buccal 

 segment has attained its full length, the dorsal cells give way 

 to a distinct epidermis and ventrally to the ganglia cells com- 

 mon to this stage. The mesoderm of the buccal segment has 

 been differentiated into a somatic and splanchnic layer, and 

 both ventrally and dorsally a distinct coelomic cavity is present 

 throughout the greater part of the segment. In the anterior 

 region, however, the cavity in this section is broken up by the 

 musculature of the median tentacle {c.vie.), but in more lateral 

 sections it is in direct communication with the cavity of the 

 head already present in stolon 2 (PL XIV, Fig. 25). 



The development of the circumoesophageal nerve ring in the 

 ectodermal tissues of the buccal segment is very similar to the 

 development of the medullary substance in the head. It first 

 appears in a stolon of a stage of development a little earlier 

 than stolon 4, and at this age it has already placed the brain 

 into direct communication with the ventral cord. In a trans- 

 verse section of the head of this stolon, passing through the 

 anterior basal parts of the median tentacle and ventral tentac- 

 ular cirri (PI. XIV, Fig. 27), the more dorsal parts of the cir- 

 cumoesophageal nerve ring {c.oes.) may be seen on each side as 

 a branch of the medullary substance extending through the 

 mass of nerve cells directly over the mesodermal tissue ; while 

 external to these branches, just as over the medullary substance 

 in the brain, are placed a number of nerve cells {c.n), some of 

 which will in the further development of the stolon add to the 

 thickening of the nerve ring, while the others will remain as 

 the nerve cells that surround these nerve fibers in the mature 

 stolon. In the ventral ectoderm of this figure, which represents 

 a part of the region of separation {r.s.), the ectodermal cells 

 have undergone a partial degeneration, thus obscuring the 

 ventral cord and the cells beneath it ; and it is just posterior 

 to this region that the nerve ring blends with the ventral cord. 



