III.] THE EARTHWORM. 247 



conditions are such as to render observation upon the ma- 

 turation of these exceedingly easy and instructive. 



The worm is hermaphrodite but not self-impregnating. 

 During copulation — which usually takes place at early 

 morning — the bodies of two individuals are brought into 

 apposition, and a transfer of ripe spermatozoa takes place. 

 These are passed into definite semiiial receptacles^ there to 

 await final deposition. During the interval which follows 

 there is secreted by the clitellum an &gg capsule or cocoon, 

 within which functionally mature ova and spermatozoa are 

 ultimately deposited. Segmentation of the fertilized ovum 

 is holohlastic, and there result two layers of cells — a more 

 rapidly dividing one, which differs from that described for 

 the Frog mainly in the absence of pigment, and a less 

 rapidly dividing yolk-laden one. The smaller cells over- 

 grow the larger ones very rapidly, and there results a simple 

 two-layered sac or gasirida which becomes ciliated ex- 

 ternally. The embryo early assumes a bilaterally sym- 

 metrical form, and as the body elongates there are de- 

 veloped, mainly if not entirely from the archenteric wall, 

 a series of paired cellular masses which become metameri- 

 cally arranged. The segmentation of the body receives its 

 initiative in the appearance of these mesoblastic somites ; from 

 the central cavities ultimately developed within them the 

 body-cavity is derived, while their walls, coming into apposi- 

 tion antero-posteriorly, give rise to the mesenteric septa. 



The nervous system arises as a thickening of the investing 

 epiblastic layer, and the supra-intestinal vessel when first 

 formed is paired. The larva continues to elongate by the 

 addition of fresh segments at its hinder end, but the sim- 

 plicity and uniformity of structure so characteristic of it, is 

 retained with but slight modifications by the adult animal. 



The clitellum, to which reference has been made, must 



