

THE EARTHWORM AND ANNELIDS IN GENERAL 185 



primitive germ cells; from here to the pointed end of the ovary 

 the cells increase in size, those toward the extreme end being 

 recognizable as eggs, each of which is surrounded by a follicle 

 of small nutritive cells. Eggs separate from the end of the ovaries 

 and pass into the body cavity. From here they enter the ciliated 

 funnel of the oviduct, thence into the egg sac (Fig. 93, R), where 

 part of the maturation processes occur. From here they either 

 pass out into the cavity of the slime tube and are conveyed from 

 the external openings of the oviduct on somite XIV to the co- 

 coon, or enter the cocoon when it passes over this somite during 

 deposition (142). 



FERTILIZATION. The eggs are penetrated by spermatozoa 

 after the cocoon is shed. At this time they have not completed 

 their maturation processes (141, 142). 



Embryology. The eggs of the earthworm are holoblastic, 

 but cleavage is unequal, the first division resulting in one large 

 and one small cell (Fig. 95, A). The second cleavage divides 

 the small cell into two equal parts and cuts off a small cell from the 

 larger (B). Because of their difference in size, the large cells are 

 called macromeres (mac.) and the small cells, micromeres (mic.). 

 Cleavage becomes irregular after the second division. Both the 

 macromeres and micromeres continue to divide, the number of 

 cells increasing to six (C), eight (D), etc. Soon a cavity, the 

 blastoccel (bl.), appears between the micromeres and macromeres, 

 and a hollow sphere recognizable as a blastula results (E). Two 

 of the larger cells of the blastula lying side by side behave differ- 

 ently from the others. They project down into the blastoccel, 

 and, by repeated divisions, give rise to two rows of small cells 

 (mes. and mm. in F and G). Because of the fact that these two 

 cells give rise to the mesoderm, they are termed mesomeres, and 

 the two rows of cells derived from them, the mesoblastic bands. 

 While the mesomeres are dividing, the blastula becomes flat- 

 tened (F), the larger cells form a plate of clear columnar cells 

 (mac.), and the small cells spread out into a thin dome-shaped 

 epithelium (mic.). 



