258 LECTURE XII. 



of other spermatozoa, which, having attached themselves to the peri- 

 phery of the cleared space, keep up a complete vortex, in which the 

 centre body is partly a passive and partly an active agent. This re- 

 markable property of spermatozoa, in performing the office of cilia, 

 is, as far as I am aware, a new fact, and one which may serve to 

 explain possibly the transference of the sperm fluid, or even of ova, 

 of various animals, along surfaces or tubes not naturally ciliated."* 



In regard to the development of the ova and embryo in the earth- 

 worm, the ova, when they are impregnated, are discharged from the 

 ovarium, pass into the abdominal cavity, and are pressed on to a re- 

 ceptacle near the anal end of the worm. Dr. A. Farre has traced the 

 development in some instances, which is so far analogous to that of 

 the Ascaris, that the embryo acquires the worm-like form before it 

 leaves the egg. It appears that some earth-worms are developed 

 before they are excluded, but this is not always the case. The 

 whole of the process, however, is complicated and obscured by a con- 

 siderable number of parasites, eggs of gregarinae and pseudo-navicellar 

 capsules, so that it is one of the most perplexing fields of microscopic 

 observation. 



The generative plienomena of the leech are less dubious. Mr. 

 Quekett and others have found that the spermatozoa are developed in 

 the testes, a, fig- 108,, in the same manner as in the earth-worm and 

 other Anellids. The ova of the leech are spherical, and exhibit, 

 when found in the ovaria, a, fig. 109., and oviducts, the usual con- 

 stituents of ova. There is a germ-yolk with a germinal vesicle and 

 spot, and a delicate vitelline membrane. When the ova are received 

 into the uterus, the germinal spot has disappeared, intimating that 

 impregnation has taken place, and a delicate reticulate chorion is 

 acquired. 



The medicinal leech is oviparous ; the fertile ova are successively 

 discharged in groups of from six to fourteen, enveloped in a nidus 

 or cocoon of mucus. The cocoon is ovate, two-thirds of an inch 

 in length, and half an inch in diameter. It lias a flocculent outer 

 surface, but is smooth and slightly tuberculate within ; a more con- 

 spicuous, though minute albuminous plug, projects inwards from 

 each pole of the cocoon. In the month of August, conical exca- 

 vations may be observed in the slime at the sides of the reservoir, in 

 each of which there is a cocoon. The formation of the cocoon is the 

 result of a curious manoeuvre. In tlie Hirtido vulgaris, when 

 the cocoon is about to be formed, the body is observed to be greatly 

 contracted, both above and below the uterus; the included part 



* CXCVI. 



