190 EMBRYOLOGY 



the PolystomidcR, also living on the gills of fishes, is very re- 

 markable. Its reproduction approaches that of Monostomum, 

 already described, for in this species also the embryo while 

 still in the body of the parent contains another embryo; 

 indeed, the latter already exhibits within itself traces of a 

 new individual, so that four generations are included one 

 within the other (Wagenee, Metschxikoff). Accordingly 

 here, as in Monostomum, the germ cells produce the new 

 generation very early; but otherwise this developmental 

 process is not very different from that of the other Trema- 

 toda. In order to understand the cause of this accelerated 

 production, one Avould have to know more accurately the 

 processes themselves, as well as the habits of the animal. 



III. CESTODA. 



The eggs of the Cestodes exhibit a close resemblance to 

 those of the Trematodes. Like these, they are composed of 

 the egg-cell proper and a number of yolk-cells ; where the 

 latter are wanting, an accessory yolk-mass corresponding to 

 them appears to be present. The eggs are surrounded by a 

 thin egg-membrane, which occasionally possesses a movable 

 lid. The development of the eggs takes place for the most 

 part in the uterus of the parent, but in many forms it occurs 

 only after the eggs are laid. In the- latter case the mem- 

 brane is thicker. 



The investigations of E. VAN Beneden and Villot on the 

 TcBiiiadoe, and especially those of Schauinsland on the 

 Bothrlocejphalidoe, have shown that the embryonic develop- 

 ment of the Cestoda takes place in a manner quite similar 

 to that of the Trematoda. 



According to Schauinsland, the development of the Bothrio- 

 cephalidcB is accomplished in two different w^ays, depending 

 upon whether the embryos are developed before, or after ovi- 

 position. The undeveloped eggs which are deposited in the 

 water are thick-shelled, operculated, and provided with a 

 large number of yolk-cells. From them emerge larvae which 

 bear a thick coat of cilia. The eggs of the second kind are 

 thin-shelled, without an operculum, and provided with only 



