III. J THE EARTHWORM. 247 



the anterior segments, and will be found described on p. 268. 

 They are somewhat complicated, chiefly owing to the con- 

 ditions of maturation of the seminal vesicles ; but the actual 

 genital glands, though exceedingly small, are well defined. 

 The ripe ovum consists of a round nucleated cell contain- 

 ing a moderate food-yolk, and invested in a vitelline mem- 

 brane. The sperm-producing cells undergo changes which 

 result in the formation of a number of filiform spermatozoa, 

 each with an elongated nucleus-bearing "head;" and the 

 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 

 deposited, the developing eggs being ultimately enclosed 

 within an albuminous fluid secreted by the so-called 

 capsulogenous glands (p. 250). Segmentation of the ferti- 

 lized 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 overgrow the larger ones very rapidly, and there 

 results a simple two-layered sac or gastrula which becomes 

 ciliated externally. The embryo early assumes a bilaterally 

 symmetrical form, and as the body elongates there are 

 developed, 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 



