S2 



THE BIOLOGY OF AN ANIMAL. 



Lumhricus, however, it goes on during gastrulation and begins 

 €ven before gastrulation. Even in the blastula stage two large 

 cells may be distinguished which afterwards give rise ^ to the 

 niesoblast and are hence called the primary mesoblastic cells. 

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

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

 face. At this period, therefore, the niesoblast forms two bands 

 of cells {mesoUast-hands) each terminating behind 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). 



Ftg. 36.— Diagrams of later embryonic stages. A^ late stage in longitudinal section, 

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

 tion ; t;, 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 ; 07% archenteron ; 

 an, anus; c<£^ coelom; cc., ectoblast; en, entoblast; ?ni, primary mesoblastic cells; 

 m", mesoblast; mh., mouth ; 7J, nervous system; s, cavity of somite; s.w, somatic 

 layer of the mesoblast, which with the ectoblast forms the somatopleure ; syA.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 



