260 LECTURE XII. 



a second, third, and fourth meridional cleft succeed each other, and 

 then the yolk is further subdivided by an equatorial cleft into nine 

 yolklets, the ninth occupying the opposite pole ; the process continues 

 until the yolk is well subdivided : when the germ-rudiment appears 

 at the germinal pole, it consists of parietal granules, spread out into 

 a layer which progressively expands to cover the germ-cells. The 

 influence of the primary germ-cells operates more partially or locally 

 upon the yolk-mass than in the Nereids; and, although the spontaneous 

 fission of the germ-cells is accompanied by total cleavage of the yolk 

 in all Anellids, that process is less regular in the aberrant Suctoria 

 than the typical orders of the class. 



The best account of the development of the medicinal leech is 

 given by Dr. E. H. Weber*, whose illustrations are copied by 

 Brandt and Ratzeburg.f The egg-mass in the uterus is cylindrical, 

 brownish, and subtransparent : it is extruded with difficulty, and can 

 hardly be taken from tlie cavity uninjured. When the mass has been 

 four hours in water, it assumes a longish oval form. Some of the 

 stages of the cleavage of the yolk have been observed in eggs in the 

 cocoon ; two of these appear to be figured by Weber.J When the 

 yolk has undergone its subdivision, and its peripheral stratum has been 

 metamorphosed into the primitive germ-layer, a large clear space or cell 

 appears in the centre, from which an infundibular canal extends to, and 

 opens upon, a part of the periphery, covered by a ciliated epithelium ; 

 the sucker, which afterwards forms the mouth, is here developed, and is 

 the first recognisable part of the future leech. The mouth, when the 

 embryo is little more than a line in length, dilates and contracts, and 

 some fine lines radiating from it ah*eady indicate the muscles sub- 

 servient to tlie act of nutrition, which consists in the embryo's 

 swallowing the surrounding albumen. The integument extends in 

 a straight line from, the mouth, along a narrow median tract of tlie 

 ventral surface of the germ-mass, to about one-third from the opposite 

 end ; and when the embryo is three lines in length, if it be placed in 

 alcoliol, and afterwards in water to restore the transliicency of the 

 gelatinous parts, the ganglionic chain may be traced along the median 

 line of this part of the integument. When the ganglions are visible 

 without chemical preparation, the mucous loops begin to appear as 

 wliitish transverse streaks. Next, the anal sucker is developed, and 

 the ceplialic end of the embryo begins to stretch forth from the yolk 

 or germ-mass. The testes, with their common sperm-duct on each 

 side, and the ovaiia and uterus, are now discernible ; but they are 



* CXCV. pp. 127—197. t CLXXXVir. taf. xxx. 



X Op. cit. t. X. figs. 2 and 3. 



