TEE GERM-LAYERS. 



149 



rapidly multiplies to form an elongated mass of cells extending at 

 each side of the archenteron. After each division the primary 

 mesoblastic cells increase in size so that up to a late stage in 



m' 



inh 



FIG. 85. Diagrams of later embryonic stages; g, embryo in long section, showing the 

 germ-layers, archenteron, and blastopore; h, the same in cross-section, showing the 

 mesoblastic bands; i, later stage; j, still later stage in longitudinal section, showing 

 the appearance of the cavities of the somites; fc, the same in cross-section; n, diagram 

 of a young worm in longitudinal section after the formation of thestomodaeum, procto- 

 daeum, and anus; I, the same in cross-section, showing the beginning of the nervous 

 system ; m, cross-section of later stage with the nervous system completely established. 

 al. alimentary canal; a?', archenteron; an, anus; cos, coslom; ec, ectoblast; en, eu- 

 toblast; m 1 , primary mesoblastic cells; m 2 , mesoblast; mh, mouth; H. nervous system; 

 s. cavity of somite; s.m, somatic layer of the me soblast, which with the ectoblast forms 

 the somatopleure; spl.m, splanchnic layer of the mesoblast, which with the entoblast 

 forms the splanchnopleure. 



development they may be distinguished from the cells to which 

 tney give rise. The two masses of mesoblastic cells gradually 

 increase in size and finally fill the segmentation-cavity. 



