REPRODUCTION AND DEVELOPMENT 



119 



it divides many times to form the somatic plate, an area of irregu- 

 larly arranged cells which is first seen dorsally but spreads over the 

 surface of the embryo until its edges meet ventrally (Fig. 70). 



Micromeres 



Mesoblast 



Micromere 

 2d derivatives 



Endoderm 



Macromere 



(a) 



Somatic 

 plate 



iVIesobiast 



Apical organ 



Endoderm 



(c) 



Nerve cord rudimen't 

 (d) 



(e) 



Somatic 

 plate 



Fig. 70. Development of polychaetes. (a) blastula, showing 

 beginnings of subdivision of 2d to form somatic plate tissue; 

 (b) section through slightly later stage ; (c) to (e) diagrams illus- 

 trating the spread of the somatic plate tissue from dorsal to 

 ventral side of the embryo, (a) after Child, 1900; (b) after 

 Wilson, 1932; (c) to (e) after Dawydoff, 1959. 



From the somatic plate are formed the ventral nerve cord and most 

 of the trunk ectoderm. 4d is known as the mesoblast and at the 

 time of gastrulation it sinks below the surface of the embryo. It 

 divides once and its descendants then bud off long chains of cells 

 which eventually form the segmented mesoderm. It is usual for 

 polychaetes to pass through a free-swimming trochophore larva 

 stage, the organization of which is basically that of a triploblastic 

 acoelomate. 



The oligochaetes, many of which are terrestrial and which 

 deposit their eggs in a nutritive fluid enclosed within a cocoon, 

 have lost the free swimming larval stage. Moreover, the pattern 

 of cleavage is often so much modified that it is difficult to detect 



