THE GERM-BANDS OF LUMBRICUS. 



EDMUND B. WILSON. 



The foundation of our knowledge of the germ-layers of the 

 Chaetopoda was laid by the classical memoir of Kowalevsky on 

 the embryology of Lunibricus rubellus and L. agricola (1871), 

 and the subsequent researches of Kleinenberg, Hatschek, and 

 other embryologists seemed sufficient for a clear understand- 

 ing of all the essential phenomena of development. An exami- 

 nation of the development of Ltnnbriciis olidus, Hoffmeister 

 (= L. fcrtidus, Dug.) has nevertheless shown that some of the 

 most important and characteristic phenomena have hitherto 

 escaped attention ; and I am led to publish this preliminary 

 article partly on account of the light that my results throw on 

 the interesting discoveries of Whitman and others in the devel- 

 opment of the Hirudinea, partly on account of their close bear- 

 ings on recent studies of the excretory system in the Vertebrata. 

 In the present communication I shall describe only the general 

 structure and mode of growth of the germ-bands, reserving for 

 a future paper an account of the early embryonic stages and a 

 detailed description of the development of organs. (See post- 

 script p. 121.) 



I. General Account. 



As in L. rnbellus (A Kowalevsky) and in L. {irapezoides) 

 comrmmis {t. Kleinenberg) the germ-bands . end behind in a 

 pair of large " mesoblasts " at the expense of which the bands 

 increase in length throughout the whole course of development. 

 At the outset these mesoblasts appear to be the sole source 

 whence new cells are added to the germ-bands. But as 

 development proceeds six other large cells are set aside as 

 builders of the bands. For the sake of brevity I shall designate 

 the eight large cells (the mesoblasts included) as teloblasts — 

 a term suggested to me by Professor Whitman. The arrange- 

 ment of the teloblasts is symmetrical with respect to the ventral 



