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 seminal receptacles^ there to 

 await final deposition. During the interval which follows 

 there is secreted by the clitellum an egg capsule or cocoon, 

 within which functionally mature ova and spermatozoa are 

 ultimately deposited. Segmentation of the fertilized ovum 

 is holoblastic, 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 gastrula 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 



