280 



General Biology 



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 micromeres are growing over the 

 macromeres. /. 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 Wilson.) 



tive germ-cells are found. The 

 ova increase in size toward the 

 extreme end, where the germ- 

 cells are distinctly rec- 

 ognizable as eggs. Each 

 egg is surrounded by a follicle 

 ( ) of 



nutritive cells. The eggs sep- 

 arate from the end of the ovary 

 and dropping into the body- 

 cavity, pass into the ciliated 

 end of the oviduct which leads 

 to the egg-sac where part of 

 the maturation takes place. 

 From here they either pass out 

 into the cavity of the slime- 

 tube and are conveyed from the 

 external openings of the ovi- 

 duct in somite 14 to the cocoon, 

 or they enter the cocoon itself 

 when it passes over this 

 somite during deposition. The 

 eggs are actually fertilized by 

 the spermatozoa after the 

 cocoon is shed and before the 

 egg has completed its matura- 



tion process. 

 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 

 macromeres, resulting in a blastula. 



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

 blastocoele. 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 meso- 

 blastic bands. During the time these bands are forming, the blastula 

 becomes flattened, the larger cells form a plate of clear columnar cells, 

 and the small cells spread out into a thin dome-shaped epithelium. 



The mesomeres lie toward the posterior end of the blastula, and the 



