374 Ralph S. Lìllie 



migbt indeed be more aecurately described as consistiug of a conti- 

 nuoiis mass of undifferentiated protoplasm, contaiiiing large nuclei 

 from which smaller nuclei are divided off anteriorly to form the 

 nuclei of the differentiatiug mesoderm. The terminal cells cannot 

 thus strictly speaking be reg-arded as possessing au iudividuality 

 which persista through developmeut. Described objectively the con- 

 ditions are simply as foUows: the extreme posterior mesoblastic 

 region coutains large clear nuclei in process of active multiplication; 

 the nuclei that remain in this position retain their embryonic 

 charaeter, while those which are given off anteriorly become smaller 

 and more chromatic (to describe simply the change in appearauce), 

 and form the nuclei of the newly differentiating tissues. Cellular 

 demarcations are in fact difticult or impossible to observe in the 

 early mesoderm of Arenicola; the term mesenchyme is therefore 

 perhaps more accurate in that it does not imply a definite epithelial 

 arrangement of the cells — a condition which is never found in the 

 growing zone of this species. 



The undifferentiated portion of the mesoderm is of very limited 

 extent (Piate 22 and 23, Figs. 9, 10, 29—31); almost immediately in 

 front of the teloblastic region the mesenchjme cells become arranged 

 to form the primitive septum bounding the newly forming somite; 

 while the cells applied to the body wall in the same region show 

 fine muscle-fibrils in their interior — the precursors of the longi- 

 tudinal muscles of the body-wall (Piate 23, Figs. 13, 14, 21, 24), 

 The limits of the somites are thus defined very early; when first 

 formed they are very short (Piate 22—24, Figs. 9, 10, 29, 32); tliey 

 soon elongate, and attain the normal proportions a short distance 

 in advance of the growing zone. 



The ectoderm and mesoderm of the growing zone thus agree 

 in the })ossessìon of large undifferentiated cells containing large 

 nuclcolated nuclei, which form the posterior termination of the 

 growing region and represent the teloblastic or undifferentiated 

 embryonic region of the elongating embryo. The fact that the cells 

 of this region are indefinite in number and arrangement — differing 

 markedly in this respect from the sharply individualized teloblasts 

 of Clepsine (Whitman, 1887) and Lumbncus (Wilson, 1889) — is net 

 in any degree ineonsistent with tlicir being described as teloblastic. 

 Whetlier the terminal growing region is constituted of one celi or 

 of several cells seems indeed to be immaterial. The distinctive 

 characteristic of teloblastic growth is the presence of a terminal 



