146 THE BIOLOG Y OF AN ANIMAL. 



somites a quantity of spermatozoa are added from the seminal 

 receptacles where they have been stored since the time of copu- 

 lation when they were obtained from another worm. The gir- 



dle is next stripped forwards over the 

 ov 



anterior end and is finally thrown com- 

 pletely off. As it passes off its open 

 ends immediately contract tightly to- 

 gether, and the girdle becomes a closed 

 capsule (Fig. 83, c) containing both ova 

 and spermatozoa floating in a nutritive 

 fluid or milk. The membrane soon as- 



FIG. 83. Egg capsule enlarged 5 

 diameters; a few eggs enlarged Slimes a light yellowish Or brown Color, 



spermatozoa (these are somewhat protect the developing embryos. The 



too large). u -f ? TUT T 



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. 



EMBRYOLOGY. 



Fertilization. The spermatozoa swim actively about in the 

 fluid which fills the capsule, approach the ova, and attach them- 

 selves to the latter by their heads in large numbers. The tails 

 still lash actively to and fro, and the spermatozoa appear to 

 make vigorous efforts to bore their way into the ova. Sooner 

 or later a single spermatozoon forces its head into each ovum, 

 leaving outside its tail, which soon loses the power of movement 

 and finally dies. When inside the ovum the head becomes the 

 centre of a star-shaped structure formed by a radiating arrange- 

 ment of the vitellus in its neighborhood. This is known as the 

 male pronucleus. 



Meanwhile the germinal vesicle undergoes great changes, which 

 finally result in the formation of a second star-shaped structure 

 called the female pronucleus. Lastly the male and female pro- 

 nuclei travel towards one another and finally fuse completely to- 

 gether, forming a single body known as the segmentation-nucleus. 



The whole process just described is known as fertilization or 

 impregnation of the ovum, and the product is called the fertilized 

 ovum. The latter is essentially similar in structure to the origi- 



