THE ORIGIN OF THE MESOBLAST-BANDS IN 



ANNELIDS. 



EDMUND H. WILSON. 

 I. 



Since the publication of Salensky's memoirs on the formation 

 of the germ-layers in various Polychaeta {Psygmobranchus, 

 Nereis, Pileolaria, Terebella, Ariciay- it has been generally 

 accepted that the mesoblast-bands ("secondary mesoblast ") 

 of annelids arise in some cases by direct proliferation from 

 the ventral ectoblast, and often without the agency of pole-cells, 

 or teloblasts ("primary mesoblasts, or pro-mesoblasts "). "Par- 

 tout," he says, "ou j'aireussi a observer les jeunes stades de 

 revolution du mesoderme somatique, ce feuillet, a son debut, 

 consistait en un epaississement ectodermique qui, sous forme de 

 deux bandelettes, regne suivant I'axe longitudinal du corps ; ce 

 n'est que dans le cours du d^veloppement qu'il se separe de 

 I'ectoderme.''^ This result was in harmony with Kleinenberg's 

 ea/'er studies upon Ltunbricus)^ it received, apparently, a com- 

 plete confirmation through the same author's splendid study of 

 Lopadorhy^icJius,^ a work that seemed at the time to place the 

 ectoblastic origin of the entire mesoblast in a number of the 

 marine annelids beyond the possibility of doubt. 



Yet this conclusion in one sense only served to render the 

 general subject of mesoblast-formation in annelids less intel- 

 ligible than ever. It had been demonstrated by Kowalevsky,^ 

 Hatschek,^ and Whitman," that in some cases the mesoblast 

 first appears in the form of a pair of large cells (teloblasts), by 



1 Arch, de Biol, III., IV., VI. 



2 Arch, de Biol., VI., p. 6i8. 



8 Quart. Jour. Mic. Sci., XIX., 1879. 

 < Zeitsch.f. wiss. Zoo/., XI.IV., 1886. 

 5 Jl/em. Acad. Imp. St. Petersburg, XVI., 1871. 

 / 6 ^r/,_ zool. hist. Wien., I., 1878. 



/ 7 Quart, your. Mic. Sci., XVIII., 1878. 



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