304 MENSCH. [Vol. XVI. 



The mesoderm beneath this thickened tissue has already- 

 undergone some changes. Posterior to the medullary sub- 

 stance and beneath the area in which the spindle cells blend 

 with the brain cells, a new growth of mesodermal tissue {c.me.) 

 has made its appearance and is pushing up toward the base of 

 the median tentacle. This tissue results as an outgrowth from 

 the mesoderm of this segment and will form the musculature 

 of the dorsal median and the other tentacles of the head. 



A transverse section of the head of a stolon of this age, pass- 

 ing through the plane G of the figure just described, is rep- 

 resented in PL XIV, Fig. 24. [This section is not exactly- 

 transverse, and ventrally includes parts of the anal segment 

 and caudal cirrus {c.c.).'\ Dorsally the brain cells (cji.) form a 

 thick mass of tissue which projects considerably- beyond the 

 lateral margins of the segment. On either side of the plane 

 occupied by the eyes, the anterior pair of which are quite well 

 developed, the brain cells are uniformly distributed, and along 

 the external border so irregularly placed as to give no indica- 

 tion of the presence of an epidermal formation. The medul- 

 lary substance (c.b.) appears as a small area immediately 

 overlying the faint dorsal muscle fibers {in.d.) and does not 

 project beyond the lateral border of the mesoderm. 



PI. XIV, Fig. 25, represents a section through the posterior 

 part of the head of stolon 2 taken in the plane H, Fig. 23. In 

 this region the cortical substance is less prominent than in the 

 preceding section, and the ganglion cells {c.n^ are less abun- 

 dant. Along the dorsal margins, as well as laterally, the nuclei 

 of these cells are again regularly arranged and seem to form a 

 layer of tissue that is directly continuous with the epidermis 

 over the parapodia (/.). The medullary substance {c.b) forms 

 a narrow plate in this region and is also less prominent than in 

 more anterior sections. Laterally the cerebral tissue in this 

 region gradually blends with the less thickened ectodermal 

 tissue of the first setigerous segment, and as yet shows no 

 signs of the circumoesophageal nerve ring. 



Beneath the cerebral tissue, on either side of the area of 

 medullary substance, a small cavity (coeJ) has made its appear- 

 ance. This cavity is the forward extension of the coelomic 



