THE EARTHWORM 



279 



OOGENESIS 



The egg-mother cells are found in the ovary in various stages of 

 growth, beginning at the basal end of each ovary with the most primitive 



germ-cells, the ova increasing 

 in size toward the extreme 

 end, where the germ-cells are 

 distinctly recognizable as eggs. 

 Each egg is surrounded by a 

 follicle ( ) of nutritive 



cells. The eggs separate from 

 the end of the ovary dropping 

 into the body-cavity, then pass- 

 ing into the ciliated end of the 

 oviduct which goes to the egg- 

 sac where part of the matura- 

 tion takes place. From here 

 they either pass out into the 

 cavity of the slime-tube and are 

 conveyed from the external 

 openings of the oviduct in 

 somite 14 to the cocoon, or they 

 enter the cocoon itself when it 

 passes over this somite during* 

 deposition, the eggs actually 

 being fertilized by the spermat- 

 ozoa after the cocoon is shed, 

 and before the egg has com- 

 pleted its maturation process. 



H. 



K. J. 



Fig. 171. 



Segmentation and early stages of development of 

 Lumbricus. A, B, C, D, successive stages of 

 segmentation. E. Blastula stage. F. Com- 

 mencement of invagination ; the macromeres 

 form a flat plate on the ventral side. G. An embryo 

 somewhat younger than F viewed from above, show- 

 ing the mesomeres and mesoblast rows derived from 

 them. H. Gastrula stage viewed from below, -show- 

 ing the wide oval blastopore bounded by macromeres ; 

 at the sides the rficromeres are growing over the 

 macromeres. J. Later stage, showing the elongated 

 blastopore and the further overgrowth of the macro- 

 meres by the micromeres. K. Optical longitudinal 

 section through a later stage after the closure of the 

 blastopore. bp, blastopore ; ec, ectoderm ; en, endo- 

 derm ; ent, enteron ; mac, macromeres ; mes, meso- 

 blast ; mic, micromeres; mm, mesomeres. (From 

 Bourne after Wilsonl) 



EMBRYOLOGY 



The egg of the earthworm is holoblastic (Fig. 171) although cleav- 

 age is unequal, the first division resulting in one large and one small cell. 

 The second cleavage divides the small cell into two equal parts but 

 cuts off only a small portion from the larger one. The small cells are 

 called micromeres and the large ones macromeres. Cleavage is very 

 irregular after this second division. The micromeres are the animal cells 

 and the macromeres the vegetative cells. 



A cavity, the blastocoele, soon forms between micromeres and mac- 

 romeres resulting in a blastula. 



Two of the larger cells of the blastula project down into the blas- 

 tocoele. These continue dividing and form two rows of small cells from 

 which the mesoderm is to form. They are therefore called mesomeres, 

 while the two rows formed from them are known as mesoblastic bands. 

 During the time these bands are forming the blastula becomes flattened, 



