192 



AN INTRODUCTION TO ZOOLOGY 



known as the teloblasts, pole cells or primary mesoderm cells. The 



blastula now commences to alter in shape, and from being spherical 

 gradually becomes elongated and flattened, till it forms a flattish 



RM, 



M. 



FIG. 61. Diagrams of the development of Lumbricus, mainly adapted 



from Wilson. 



L and II., early segmentation stages ; III., section through the blastula showing the primary 

 mesoderm cell which has divided ; IV., section of beginning of invagination, macromeres forming a 

 flat plate, more mesoderm cells formed ; V., view of half an embryo during invagination ; VI., 

 section through gastrula after obliteration of blastocoel and closure of blastopore ; VII., ventral 

 view of embryo at a later stage showing stomodceum and germ bands ; VIII., transverse section of 

 lower part of an embryo showing the ccelom arising as a split in the mesoderm. 



Bl., blastocoel ; Bp., blastopore ; C., ccelom ; E., entoderm ; EC., ectoderm ; En., enteron ; 

 M., mesoderm ; Ma., macromeres ; Mi., Micromeres ; N., cells giving rise to nerve bands ; Ne., 

 cells giving rise to nephridial bands ; P.B., polar bodies ; P.M., primary mesoderm cells ; S., 

 stomodceum. 



oval cake, the small cells on the upper surface and the large ones on 

 the flat under face. Simultaneously with this alteration in shape 

 the teloblasts have sunk more and more into the embryo, until they 

 pass right into the blastoccel, and at the same time they have each 

 undergone a series of unequal divisions. These divisions result in 



