THE OVUM. 43 



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, etc. 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 ener- 

 getically 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. 45). 

 The ova are developed from the primitive germinal cells which 

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

 number, and a nucleated peripheral layer of each cell becomes 

 separated from the central part, which also contains nuclei. 

 This latter part next divides into numerous cells, of which one 

 eventually forms the ovum, and the remainder constitute a mass 

 of cells adjoining it as in Bonellia (fig. 16). This mass of cells 

 eventually disappears, and is probably employed in the nutrition 

 of the ovum. 



The ovaries of the Leech appear to belong to the tubular 

 type in that the ova are not formed from part of the epithelium 

 lining the body cavity ; but if, as seems probable, the true 

 affinities of the Leeches are with the Chaetopoda, the investment 

 of the ovaries must be of a secondary nature. It should be 

 noted that the ova are not, as in the ordinary tubular ovary, 

 developed from the epithelium lining the ovarian tube. 



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. 



