g2 TUB BIOLOGY OF AN ANIMAL. 



Lwnlricus, however, it goes on during gastralation and begins 

 even before gastrulation. Even in the blastula stage two large 

 cells may be distinguished which afterwards give rise to the 

 mesoblast and are hence called the primary mesoUastie cells. 

 They soon bud forth smaller cells into the segmentation-cavity, 

 and as the blastula flattens they themselves sink below the sur- 

 face At this period, therefore, the mesoblast forms two bands 

 of cells (mesoblast-bands) each terminating beliind in the large 

 mother-cell or pole-cell. Throughout the later stages the pole- 

 cells continue to bud forth smaller cells which are added to the 

 hinder ends of the mesoblast-bands (Figs. 35, 36). 



ec 



n 



FIG. 36. Diagrams of later embryonic stages. A, late stage in longitudinal section, 

 showing the appearance of the cavities of the somites ; B, the same in cross-sec- 

 tion ; E, diagram of a young worm in longitudinal section after the formation of 

 the stomodeeum, proctodeeum, and anus; C, the same in cross-section, showing 

 the beginning of the nervous system ; D, cross-section of later stage with the 

 nervous system completely established, al, alimentary canal ; ar, archenteron : 

 on, anus; cce, coelom; ec, ectoblast; en, entoblast; m 1 , primary mesoblastic cells; 

 m", mesoblast; m/i, mouth; n, nervous system; *, cavity of somite; s.m, somatic 

 layer of the mesoblast, which with the ectoblast forms the somatopleure ; p!.m, 

 splanchnic layer of the mesoblast, which with the entoblast forms the splanch- 

 nopleure. 



After each division the pole-cells increase in size, so that up 

 to a late stage in development they may be distinguished from 



