188 



EMBRYOLOGY 



II. POLYSTOMID^. 



The eggs in the Polystomidae also are composed of the 

 egg-cell proper and yolk-cells (Fig. 93). Their egg-mem- 

 brane is provided with an operculum, and occasionally with 

 a long_ filiform, and twisted process, which serves for the 

 attachment of the eggs (Diplozoön). The course of develop- 

 ment is simpler than in the Distomidse, for the embryo 

 while still in the egg-membrane attains nearly the form of 



m 



Pig. 93.— Egg of Mict-ocotyle Mormyn. 

 Within its operciilated shell lies an egg- 

 cell surrounded with yolk-cells (after 

 Lorenz, from Hatschek's Lehrbuch). 



Fig. 91. — Embryo of Polystomum 

 iiitegerrimum,, shortly after hatch- 

 ing (after Zei.leb). 



the parent (Gyrodactylus), or at least passes through only a 

 single metamorphosis, not an alternation of generations 

 (heterogony). 



The early development has been but little studied. We 

 are best acquainted with it (Zeller, Nos. 16 and 17) in the 

 case of Polystomum integerrimum, which inhabits the urinary 

 bladder of the frog. The eggs of this species are voided into 

 the water, where cleavage soon begins. The result of this 



