THE BIOLOGY OF AN ANIMAL. 



Lnmbricus, however, it goes on during gastrulation and begins 

 even before gastmlation. Even in the blastula stage two large 

 <jells may be distinguished which afterwards give rise to the 

 mesoblast and are hence called the primary mesollastic 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-lands) each terminating behind in the large 

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

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

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



,m 



en 



inh 



FTG. 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 stomodaeum, proctodaeum, 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, a?, alimentary canal ; ar, archenteron ; 

 em, anus; co?, coelom; cc, ectoblast : en, entoblast ; m 1 , primary mesoblastic cells; 

 ?n 2 , mesoblast; mfc, mouth; n, nervous system; s, cavity of somite; s.m, somatic 

 layer of the mesoblast, which with the ectoblast forms the somatopleure ; sp?./, 

 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 



