THE EARTHWORM 



37 



It now becomes elongated antero-posteriorly and flattened 

 from above downwards, so as to form an oval plate with 



Fig. 8 



Segmentation and early stages of development of Lumbricus, after E. B. Wilson. 

 A, B, C, D, successive stages of segmentation. E. Blastula stage. F. Commence- 

 ment of invagination ; the macromeres form a flat plate on the ventral side. G. An 

 embryo somewhat younger than F viewed from above, showing the mesomeres and 

 mesoblast rows derived from them. //. Gastrula stage viewed from below, showing the 

 wide oval blastopore bounded by macromeres ; at the sides the micromeres are growing 

 over the macromeres. /. Later stage, showing the elongated blastopore and the further 

 overgrowth of the macromeres by the micromeres. K. Optical longitudinal section 

 through a later stage after closure of the blastopore. J>p, blastopore ; ec, ectoderm ; 

 en, endoderm ; ent, enteron ', mac, macromeres ; mes, mesoblast ; tnic, micromeres ; mm, 

 mesomeres. 



rounded edges. The cells on one side, the lower side, are 

 larger than those on the upper side, and become tall and 



