DEVELOPMENT OF EARTHWORM 



255 



exhibits considerable variation even 

 within the hmits of a species. 



In about twenty-four hours a 

 nearly spherical, one-layered blasto- 

 sphere or blastula is formed. It con- 

 sists of only about thirteen cells. 

 During the next twenty-four hours 

 the cells increase in number rapidly, 

 but the blastula remains one-layered. 

 Two cells lying together do not take 

 part in this division ; they are rather 

 larger than the rest, and their inner 

 ends project into the cavity, and are 

 soon cut off as daughter-cells. Gradu- 

 ally the large cells still undergoing 

 division begin to sink in, and at last 

 are quite included in the cavity (Fig. 

 137 (2)). Thus there arise two parallel 

 rows of cells within the blastula, and 

 these define the longitudinal axis of 

 the embryo. This is the beginning of 

 the mesoblast which forms all the 

 muscles of the trunk, and which thus 

 takes origin from two primary meso- 

 blasts. 



After five to six pairs of secondary 

 mesoblasts have been formed, the 

 blastula begins to flatten, and to 

 elongate, becoming an oval disc. 

 The cells of the lower surface be- 

 come clearer, and the endoderm is 

 thus defined. The cells of the upper 

 surface are smaller, and become very 

 much flattened ; they compose the 

 ectoderm. The mesoblasts lie side 

 by side near one end, forming two 

 rows extending forwards and down- 



FiG. 137. — Stages in the develop- 

 ment of earthworm. — After Wilson. 



I. 



2. 



3- 



Two-celled stage ; p.c, polar bodies. 



Blastula ; M., primary mesoderm cell. 



Gastrula stage ; Ec, ectoderm ; En., 

 endoderm in process of being covered 

 by the small ectoderm cells. Note the 

 widely open blastopore ; M., mesoderm 

 cells. 



Longitudinal section in late gastrula stage, 

 showing germ-bands ; ec, ectoderm ; 

 en., endoderm ; M., mouth ; St., stomo- 

 daeum ; m., primary mesoderm cells; 

 Nb., neuroblasts ; n.c, nerve-cord ; N., 

 nephridioblasts ; ms., mesoderm bands ; 

 npc, incipient nephridia. 







ms 



^^m 



