226 



INVERTEBRATE MORPHOLOGY. 



end of the body and giving rise by division in a transverse 

 plane to rows of cells, the mesoblast bands. Such a pre- 

 cocious segregation of the inesoderm also occurs in the Oli- 

 gocliseta. In a Lumbricus embryo there may be seen near 

 the posterior extremity of the body the two mesoblasts (Fig. 



105, ra), lying one on each side 

 of the middle line, with the meso- 

 blast-bauds (mb) extending for- 

 wards from them. A little in 

 front of them and on either side 

 may be seen another cell (nb), 

 giving rise to a band extending 

 anteriorly, which later on will 

 become differentiated into the 

 ventral nerve-cord, the cells which 

 give rise to it being neuroblasts j 

 while a little behind and exter- 

 nally to these, on either side, two 

 TKUIOK EXTREMITY OF JLMBRYO 



OF Lumbricus (after E. B. WILSON), other cells (we and a;) occur, giving 



rise likewise to germ-bands, whose 

 further fate is undecided, though 

 it seems probable that the inner of 

 the two bands gives rise to the 

 nephridia, the cells being nephro- 

 blasts. Thus from a small number 



of cells the entire nervous system, with the exception of the 

 suprao3sophageal ganglion (which arises as a local thickening 

 of the ectoderm, comparable to the apical thickening of the Tro- 

 chophore), the nephridia and all the other niesodermal tissues 

 arise, the precocious segregation of these organs being carried 

 to an extent only equalled in the Hirudinea. Indications of 

 it, however, are found in the Polychseta, not only in the meso- 

 blasts but also in a layer of cells occupying the ventral 

 surface of the embryo, and forming the so-called ventral plate, 

 from which the ventral nerve-cord, the nephridia, and some of 

 the musculature seem to arise. A reduction of the number 

 of cells constituting this ventral plate to the smallest number 

 consistent with a bilateral symmetry, that is, to two for each 



FIG. 105. SURFACE VIEW OF Pos- 



ec = ectoderm. 

 TO = niesoblnst. 

 mb mesoblast-band. 

 nb = lieu rob last. 

 ne nepbroblast. 

 x = lateral teloblast. 



