26 PLATYELMINTHES. 



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 Macro- 

 stomum. 



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 Beueden 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 represented 

 by a lower specialised 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 of the ovarian tube, which, instead 

 of retaining their vitality as in Prorhyucus, 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 

 the cells which form the yolk-gland give rise to yolk particles 

 which enter the ovum, instead of to a layer of yolk-cells surrounding 

 the ovum. 



Amongst the marine clendrocefilous Turbellarians the ova are formed in 

 separate sacks widely distributed in the parenchyma of the body between 

 the alimentary diverticula. In these the ova undergo their complete develop- 

 ment, without the intervention of yolk-glands. 



The ovaries of the Nemertines more nearly resemble those of the 

 marine Dendroccela than those of the Rhabdoccela. They consist of a series 

 of sacks situated on the two sides of the body between the prolongations 

 of the digestive canal. The eggs are developed in these sacks in a perfectly 

 normal manner, and in many cases become rilled with yolk-spherules which 

 arise as differentiations of the protoplasm of the ovum. The protecting 

 membranes of the ova have not been accurately studied. In some cases ' 

 two membranes are present, an internal and an external. The former, 

 immediately investing the vitellus, is very delicate : the external one is 

 thicker and hyaline. 



The constitution of the female generative organs of the Trematodes 

 was first clearly ascertained by von Siebold (No. 12). He originally, 



1 Amphipoius lactiflorius and Nemertesgracilis. M 1 ' In tosh. Monograph on British 

 Kay Society. 



