3IO MENSCH. [Vol. XVI. 



ment of the setigerous series that retains the purely embryonic 

 characters, the segments preceding it, in very young stolons at 

 least, as soon as formed undergoing a sufficient differentiation 

 to make the distinction quite marked. This is particularly evi- 

 dent in the stolons represented in PI. XIII, Fig. 17, and even 

 in PL XIV, Fig. 23, where a new segment has just been formed; 

 and in these stolons it may be safely said that the anal segment 

 forms exclusively the region of elongation. In stolons 2 and 3, 

 where elongations of the stolon by the addition of new segments 

 are very active, the tissues of the young segments anterior to 

 the anal have, in longitudinal section, an embryonic appearance 

 similar to that of the anal segment, and in these segments the 

 different tissues, as well as the external constrictions in the 

 mid-region of some of these segments, indicate the possibility 

 of an increase in the number of segments by division of some 

 of the newly formed setigerous segments. Later on, however, 

 when the formation of new segments becomes less rapid, the 

 embryonic tissues are again confined to the anal segment (PI. 

 XIII, Fig. 26), and in all stages of development the tissues of 

 the anal segment appear to be active centers from which are 

 developed the new segments of this region. The center of 

 growth in the anal segment is similar to that in the region of 

 proliferation in the last segment of the parent stock, and in ex- 

 amining the anal segments of stolons in different stages of 

 development it will be found that the embryonic tissues are 

 retained as long as new segments are added to the stolon. 



Malaquin (9) regards the formation of new stolons in the 

 posterior part of the stolon as taking place in a region anterior 

 to the anal segment which he calls the Zoonite formateur. In 

 describing the formation of new segments in the stolon of 

 Myrianida he says : "II semble done tres probable que ce 

 phenomene est bien constant et que la production de stolons se 

 fait chez la Myrianide dans la zone terminale meme d'accroisse- 

 ment de I'adulte, c'est-a-dire dans la zone precedant immediate- 

 ment le pygidium, et que j'ai designee deja sous le nom de 

 Zoonite formateur. ... La zone active de proliferation, le 

 zoonite formateur, est done bien distinct du pygidium, en tant 

 que zoonite ; si, en effet, le pygidium avait le pouvoir de former 



