THE EARTHWORM 211 



Egg-capsules are found buried in the soil and under stones 

 and boards and other protected places. They are often mis- 

 taken for seeds of some plant, which accounts for their iden- 

 tity being unknown to most people. 



From the drawing we get an idea of the relative size of 

 the egg (Fig. 103, B) and the spermatozoon (Fig. 103, C). In 

 the case of animals which produce a large, yolk-filled egg, 

 e.g. birds, the difference in size between the egg and sper- 

 matozoon is far greater. The spermatozoon of all animals is 

 microscopic in size. 



Maturation. The phenomena of fertilization in the egg of 

 the earthworm are not so well known as they are in a near 

 relative, the sandworm, Ne'reis (Fig. 107). The series of fig- 

 ures (Fig. 104, J, /?, C, D, E, F, maturation or egg-ripening 

 in Nereis) illustrates in a general way the phases through 

 which the eggs of animals pass before they begin to develop. 

 Fig. 104, A shows the egg fully grown and ready to receive 

 the spermatozoon. Among the things to be noted are the 

 egg-membrane and the nucleus containing the germinal spot. 

 Imbedded in the fine granular protoplasm (cytoplasm) about 

 the nucleus are the yolk-granules, destined to nourish the 

 developing embryo, and some large fat-globules. 



At about the time of penetration of the egg by the sper- 

 matozoon (Fig. 104, B) the egg-nucleus begins to lose its out- 

 Une because of the encroachment of a spindle-shaped structure 

 ^ormetl from minute fibers in the cytoplasm. At either pole 

 of the spindle is an extremely small granule. These are 

 illed centrosomes (central bodies). They appear to dominate 

 the formation of the spindle and its movements after forma- 

 tion. The spindle draws from the egg-nucleus a definite 

 number of small, rod-like bodies called chromosomes (colored 

 odies). They are so called because in preparations of thin 

 ices of the egg these little bodies stain in certain dyes more 

 b illiantly than any other part. 



