34 DISCOPHORA. 



constitute a germinal epithelium (fig. 15). Very frequently (Aphro- 

 dite, Arenicola), as is so common in other types, these tracts of ger- 

 minal cells surround the blood-vessels. In some cases the germinal 

 epithelium thickens to form a compact organ, for which the outer- 

 most cells may form a more or less definite membranous covering 

 (Oligochseta etc.). The ova are formed by the enlargement, accompanied 

 by other changes, of these germinal cells. During their early develop- 

 ment the ova are frequently surrounded by a special capsule, which is 

 often stalked, and provided at its attachment with a large micropylar 

 aperture. In Aphrodite and Polynoe this arrangement, which is clearly 

 connected with the nutrition of the ovum, is very easily seen. The 

 ovum is dehisced into the body cavity by the bursting of its capsule or 

 the rupture of the stalk. The capsule is always eventually thrown 

 off; but a vitelline membrane is frequently developed after the 

 detachment of the ovum into the body cavity. The vitelline mem- 

 brane of Spio and other Polycha-ta is provided with an equatorial ring 

 of ampulliform vesicles. 



DISCOPHORA. 



(36) H. Dorner. "Ueber d Gattung Branchiobdella. " Zeit. f. wiss. Zool., 

 Vol. xv. 1865. 



(37) R. Leuckart. Die menschlichen Parasiten. 



(38) Fr. Leydig. "Zur Aiiatonaie v. Piscicola geometrica, etc." Zeit. f. wiss. 

 Zool., Vol. i. 1849. 



(39) C. O. Whitman. "Embryology of Clepsine." Quart. J. of Micr. Sci., Vol. 

 xvin. 1878. 



The ovary of the Discophora is formed of a mass of cells en- 

 veloped in a membranous sack. In Branchiobdella there is placed 

 in the central axis of these cells a column of nucleated protoplasm 

 from which the cells themselves are budded off. The development of 

 the ovum takes place by the enlargement &c. of one of the peripheral 

 cells, which eventually bursts the wall of the sack and is freely dehisced 

 into the body cavity. 



In most other Leeches (except Piscicola and its allies) there is 

 found a more specialized arrangement of the same nature as in 

 Branchiobdella. There are one or more coiled egg-strings which lie 

 freely in a delicate sack continuous with the oviduct. Each egg-string 

 is formed of a central rachis and of a peripheral layer of cells 1 . The 

 ova are formed by the enlargement of the peripheral cells accompanied 

 by a deposition of food-yolk. Food-yolk appears to be formed in the 

 rachis even more energetically than in the protoplasm of the ova. 

 When ripe the ova fall into the ovarian sack. 



In Piscicola the development of the ovum is somewhat pecu- 

 liar but resembles in certain respects that of Bonellia (p. 36). 

 The ova are developed from the primitive germinal cells which 

 fill up the ovarian sack. The nuclei in these cells increase in number, 



1 The rachis is stated by Whitman (no. 39), and other observers to be formed of 

 nucleated protoplasm, but further investigations on this point are still required. 



