32 PLATYELMINTHES. 



than in the preceding types. In most of the Rhabdocoela and 

 fresh-water Dendrocoela these organs resemble in their funda- 

 mental characters those of the Trematodes and Cestodes. There 

 are present a paired or single ovary and a paired yolk-gland. 

 The general arrangement of the organs is shewn in fig. 9. 



The blind end of the ovaries is usually (Ed. van Beneden, etc.) 

 stated to be formed of a polynuclear protoplasmic basis, but 

 Hallez (No. 10) has recently insisted that, even at the extreme 

 end of the ovary, the germinal cells are quite distinct, and not 

 confounded together. 



With one or two exceptions the yolk-cells secreted by the 

 vitellarium retain their vitality till they are swallowed by the 

 embryo, after the development of its mouth. The few not so 

 swallowed become disintegrated. They are granular nucleated 

 cells, and, as was first shewn by von Siebold, are remarkable for 

 exhibiting spontaneous amoeboid movements. 



Very important light on the nature of the vitellarium is 

 afforded by the structure of the generative organs in Prorhyncus 

 and Macrostomum. 



In Prorhyncus there is no separate vitellarium, but the lower 

 part of the ovarian tube functionally and morphologically 

 replaces it. The ovum becomes surrounded by yolk-cells, 

 which according to Hallez (No. 10) retain their vitality for a 

 long time. According to Ed. van Beneden yolk-spherules are 

 formed in the protoplasm of the ovum itself, in addition to and 

 independently of the surrounding yolk-cells. In Convoluta 

 paradoxa a special vitellarium is stated to be absent ; though a 

 deposit of yolk is formed round the ovum (Claparede). 



In Macrostomum again the yolk-glands are at most repre- 

 sented by a lower specialized part of the ovarian tube. The ova in 

 passing down become filled with yolk-spherules. According to 

 Ed. van Beneden these spherules are formed in the protoplasm 

 of the ovum itself; but this is explicitly denied by Hallez, who 

 finds that they are formed from the lining cells ot the ovarian 

 tube, which, instead of retaining their vitality as in Prorhyncus, 

 break up and form a granular mass which is absorbed by the 

 protoplasm of the ovum. 



In Prostomum caledonicum (Ed. van Beneden) the generative 

 organs are formed on the same plan as in other Rhabdoccela, but 



