300 MENSCH. [Vol. XVI. 



the second segment presents characters common to the last 

 segment of each stolon, and as the segment following already 

 shows ectodermal thickenings indicative of a head formation, 

 this segment may be regarded as the anal segment. 



PI. XIV, Fig. 21, represents a transverse section through the 

 posterior part of the first segment of stolon A, PI. XIII, Fig, 6, 

 and indicates a condition less embryonic than that presented 

 by the two preceding segments. The ectoderm in this section 

 is very little thickened dorsally, though in the anterior parts 

 of this segment, as will be seen in another section, the thicken- 

 ing is considerable. Laterally the ectoderm has pushed out in 

 the form of small processes which form the rudiments of the 

 parapodia (/.). Ventrally the ectodermal cells have increased 

 in number, and in the region of the nerve cord the number of 

 nuclei {c.n) has increased sufficiently to suggest a region of 

 ganglion cells. The cord itself has increased in thickness and 

 presents an appearance common to the ganglionic region. The 

 mesodermal tissue has also undergone some differentiation. A 

 coelomic cavity has made its appearance dorsally, and in part 

 laterally, while the dorsal and ventral muscle bands {ni.d. and 

 m-.v)) appear much more prominent than they do in the preced- 

 ing segments. In the ventral lateral regions, however, the 

 embryonic masses of mesoderm {in.e) still exist and form quite 

 a dense undifferentiated structure. 



Sections through the second segment of stolon A show no 

 characters different from those presented in the segment just 

 described, save a slight decrease in the thickness of the dorsal 

 ectoderm ; while sections of the third segment of this embry- 

 onic stolon repeat almost exactly the structures presented by 

 PI. XIV, Fig. 20. 



In a longitudinal section (PI. XIII, Fig. 17) this stolon (St. 

 A) appears as quite a distinct stolon. Anterior to the plane 

 of Fig. 21 {E), the dorsal ectoderm of the first segment {c) 

 has become considerably thicker, and spindle-shaped cells 

 similar to those which appeared beneath the ventral cord 

 in the embryonic region of segment 35 are beginning to make 

 their appearance in this region. Over the anterior part of the 

 second segment the ectodermal cells are less spindle-shaped 



