Structure and Development of the Nephridia. 375 



luidifferentiated growing region, wliich retains its embiyonic cbaracter 

 throughout development and gives rise anteriorly to the differentiated 

 tissues of the developÌDg orgauism; and on this criterion the develop- 

 ment of both ectodermal and mesodermal stmctures in Arenicola is 

 tvpìcally teloblastie. The entoderm on the other band does not 

 aiiparently elongate by teloblastie growth and it does not, pro- 

 perly speaking, exhibit a metamerie structure in the same sense as 

 do the ectoderm and mesoderm. Its thin-walled posterior portion is 

 frequently drawn out at the lines of Insertion of the primitive septa 

 (as represented in Plates 23 and 24, Figs. IO, 30 — 33), and in this 

 manner partakes of the earh' metamerism; but this apparent meta- 

 merism seems to be purely incidental and imposed. upon the organ 

 from without, and therefore not iuherent in its structure. It is 

 noteworthy that in other species that shovs^ teloblastie growth the 

 intestine never seems to develop teloblastically. This fact seems 

 to afford support to those authors (such as Hatschek, Meyer, 

 Bergh, Goodrich) who believe that metamerism in annuiate ani- 

 mais is primarily mesoblastic, and that the other metamerie 

 Systems of organs bave become implicated in a purely secondary 

 raaimer. 



The large mesodermal and ectodermal cells of the growing zone 

 are present, as above described, throughout the entire period of 

 somite-formation. The ectodermal cells are evidently the lineai 

 descendents of the cells described by Child (1900) as derived from 

 the division of the "stemcells", — i. e. of the two symmetrical 

 derivatives of the somatoblast (2'^ of the second quartette of ecto- 

 . meres), which form the posterior portion of the somatic piate. 

 These cells at the commencement of the concrescenee of the somatic 

 piate form "a transverse band of seven cells lying just anterior to 

 the paratroch and extending in the same direction with it. These 

 cells are large and thick and grow rapidly after division. It seems 

 perfectly clear that we bave bere what is known as the 'growing 

 tip', i. e. that portion of the body just anterior to the paratroch, 

 which continually gives rìse to the new Segments and leads to the 

 elongation of the larva. The final results of concrescenee will be 

 first, to unite the ends of the paratroch, and then the ends of the 

 second row of cells — the growing tip" (p. 628). Child did not 

 follow the concrescenee to its completion; but from the position and 

 relations of the large ectodermal cells of the larvai growing region 

 there can be no doubt that these are the direct descendents of the 



Mittheilungen a. d. Zool. Station zu Neapel. Bd. 17. 25 



