ANNELIDA 



283 



of the mesodermal bands. These arise by the division of the 

 two cells, and by the smaller cells thus produced moving 

 away from them. This process can best be understood from 

 the figures of Lumbrictis given by Kowalevsky and Klei- 

 nenberCt (Figs. 129 and 130). The two large cells (pole cells) 

 from which the smaller cells of the mesodermal bands have 

 arisen by division are seen in the posterior part of the 

 embryo (comp. the interpretation given by Kleinenberg, p. 

 286). The mesodermal bands extend on both sides of the 

 embryo towards the mouth ; whereas they at first diverge, 

 later they move from the lateral position toward the ventral 

 surface, and now lie on either side of the median line (Fig. 

 129 i?, C). 



mes. mes. 



Fig. 130.—^, B, optical longitudinal flections of two embryos of Lumhricus 

 ruhcllus {Allolohophora fcetida [?], Vejdovsky) of different ages (after Kowalevskt). 

 bt, blastopore ; ect, ectoderm ; eiit, entoderm ; m, mouth-opening ; mes, mesodermal 

 bands ; p, pole cells of the mesoderm. 



In the figures 130 A and B, the mesodermal bands are 

 seen in side view, and their first appearance (Fig. 130 J^) 

 can be recognized. In this case they consist fi'om the begin- 

 ning, not of one, but of several, cell-rows ; but even here the 

 pole cell of each band can be seen at the posterior end. The 

 bands extend further and further, and finally acquire the 

 considerable length which is represented in the figures 130 

 B and 131. 



Together with the elongation of the mesodermal bands 

 already described, which are also, though inappropriately, 

 called germ bands, the formation of the embryo as a whole 

 has progressed fai-ther (Figs. 130 B and 131). It has en- 

 larged by the rapid multiplication of its cells, and now con- 



