PHYLUM ANNULATA 



453 



By swallowing movements the embryo at this stage, having burst 

 through the enclosing vitelline membrane, takes in the albuminous 

 fluid in the interior of the cocoon, and increases rapidly in size. 

 As the embryo elongates, the mesoderm bands become divided 

 into segments, and the subsequent history of these is essentially 

 similar to what has been already described in the case of Nereis. 

 The ectoderm is thickened on each side along the line of the meso- 



blast op 



rn.es 



Tie 7' 



FIG. 370. Early stages in the development of Lumbricus. .4, lateral view of flattened 

 blast.ula ; B, ventral view of gastrula with slit-like blastopore ; C, lateral view of later stage. 

 b/astoc. blastoccele ; blastop. blastopore ; ect. ectoderm ; mil. endoderm ; in. primary meso- 

 derm cell ; mes. mesoderm bands ; ner. cell from which the primitive nerve-cord (ne. co.) 

 takes origin ; nph. cells taking part in the formation of the nephridia ; si stomodaeum. 

 (After Wilson.) 



derm bands, and the mass of .ectoderm cells so formed becomes 

 arranged in a number of rows each originating behind in a larger 

 rounded cell or teloblast. The innermost of these rows (Fig. 370, 

 C, ner, ne. co) give rise to the ventral nerve-cord. The next two 

 rows (nph) are said by some observers to give rise to the nephridia 

 all but the funnels ; but according to others the nephridia, or at 

 least all their inner glandular portions, are of mesodermal derivation, 



