78 



THE BIOLOGY OF AN ANIMAL. 



separate. [The spermatozoa thus received are simply stored up 

 and do not perform their function until the time of egg-laying.] 

 AVhen the worm is ready to lay its eggs the glands of the 

 clitellum become very active, pouring out a thick glairy fluid 

 which soon hardens into a tough membrane and forms a girdle 

 around the body. Besides this a large quantity of a thick jelly- 

 like nutrient fluid is poured out and retained in the space be- 

 tween the girdle and the body of the worm. The girdle is 

 thereupon gradually worked forward toward the head of the 

 worm by contractions of the body. As it passes the lith somite 

 a number of ova are received from the oviducts, and between 

 the 9th and llth somites a quantity of spermatozoa are added 

 from the seminal receptacles where they have been stored since 

 the time of copulation, when they were obtained from another 

 worm. The girdle is next stripped forwards over the anterior 



end and is finally thrown 

 completely off. As it 

 passes off its open ends 

 immediately contract 



/ 



tightly together, and the 

 girdle becomes a closed 

 capsule (Fig. 33) contain- 

 ing both ova and sperma- 

 tozoa floating in a nutri- 



FIG. 33. ^4, egg-capsule enlarged 5 diameters . . 



(a few eggs, oy, enlarged to the same scale are tive tlUKl Or milk. llie 



shown near by on the right) ; B, an ovum very mem t, rane soon assumes a 

 much enlarged ; C, a spematozoon, enormously 



magnified; n, head; ?, middle piece ; t, tail. light yellowish Or 1 1'OWll 



color, becomes hard and tough, and serves to protect the c.e- 

 veloping embryos. The capsules may be found in May or June 

 in earth under logs or stones, or especially in heaps of manure. 

 Within the capsules the fertilization and development of the ova 

 take place. 



Fertilization and Embryological Development. The sperma- 

 tozoa swim actively about in the nutrient fluid of the capsule, 

 approach an ovum, and attach themselves to its surface by their 

 heads. Several of the spermatozoa then enter the vitellus (cf. 

 p. 80), but it has been proved that only one of these is con- 

 cerned in fertilization, the others dying and becoming absorbed 

 by the ovum. 



