III.] 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 body-wall an egg capsule or cocoon, 

 within which functionally mature ova and spermatozoa are 

 deposited. The developing eggs are ultimately enclosed 

 within an albuminous fluid. The so-called capsulogenous- 

 glands (p. 250) are regarded as the seat of its secretion, but 

 there are reasons for believing that they may be concerned 

 in the formation of the cocoon, usually attributed to the 

 clitellum. 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 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 



