294 MENSCH. [Vol. XVI. 



development of the head of the stolon, therefore, means the 

 appearance by a different method of such structures as are 

 present in the head of the parent stock and which have been 

 formed in the ordinary processes of development. 



Region of Proliferation. 



The changes brought about by the phenomena of stoloniza- 

 tion are already evident anterior to the chain of stolons in the 

 internal structures of the last segment of the parent stock 

 (Fig. 6, 35) in an area which I have designated as the region 

 of proliferation. In a transverse section of this region (PL 

 XIII, Fig. 15) it is at once apparent that structures closely 

 associated with the processes of stolonization have made their 

 appearance which are not present in sections of segments ante- 

 rior to the segment preceding the chain. These structures 

 consist of masses of mesodermal tissue (jn.e.), which completely 

 fill the coelome laterally and even extend into the cavities of 

 the parapodia. Dorsally and ventrally the tissue is scantier, 

 and well-defined areas of body space [coe.) are visible. In the 

 anterior parts of the segment this mesodermal tissue is almost 

 entirely absent, and a section of this region would not be at all 

 unlike a similar section through any of the preceding segments. 

 In all preceding segments the body cavity is spacious, and such 

 mesodermal masses can be observed only in a segment imme- 

 diately preceding a chain of stolons. Examined under a higher 

 magnification, this tissue will be seen to consist of cells which 

 appear embryonic in structure and are in no wise different from 

 those constituting the mesodermal structures of embryonic seg- 

 ments. Associated with this tissue are the muscles of the 

 parapodia {m.ac), while dorsally and ventrally may be seen the 

 dorsal i^n.d.) and ventral {m.v.) muscle bands, all of which 

 structures, as well as those of the dorsal {d.v.) and ventral 

 {v.v.) blood vessels, can readily be distinguished from this 

 tissue. In regions a little posterior to the section here rep- 

 resented this tissue becomes more dense, fills the entire 

 coelome cavity, and is directly continuous with similar tissues 

 of the succeeding segments. 



