242 



PLATYHELMINTHES. 



new host, where the cylindrical female ends her existence in the act of expelling 

 her eggs, while the flat female breaks up into a number of sacs, each enclosing a 

 number of ova. The eggs of the cylindrical female are probably fertilised, while 



those of the flattened form are supposed to 

 develop parthenogenetically. 



The Dicyemidae have two kinds of embryos 

 (hence the name of the group) the infusori- 

 form (Fig. 196) and the vermiform (Fig. 195) 

 which arise from the egg-like germs in the 

 central plasmic mass. They arise in indi- 

 viduate of slightly different form the indi- 

 viduals producing vermiform embryo are 

 called nematogens and are longer and thinner 

 than those producing infusoriform embryos, 

 which are called rhombogens. The rhom- 

 bogens, however, after producing a certain 

 number of infusoriform embryos become 

 nematogens and produce vermiform embryos. 

 The vermiform embryo changes directly into 

 the parent form. The infusoriform embryo 

 is very different from the parent, and its fate 

 is unknown. Very possibly it has the power 

 of making its way out of its host, and so 

 distributing the species. No males or sperma- 

 tozoa have been observed in the Dicyemidae, 

 and it has been suggested that the infusori- 

 form embryos are immature males, or that 

 they contain the male elements. Whitmtin 

 indeed states that he has found them in a 

 modified form within nematogenic adults. 

 The origin of the egg-like germs within the 

 central plasmic mass has been described as 

 a case of endogenous cell-formation; i.e. a 

 nucleus gathers round itself a certain amount 

 of protoplasm, Avhich becomes delimited, to 



FIG. 195. A-D, stages in the develop- 

 ment of the vermiform embryos of 

 Dicyema; A, of Dicyemennea eledones 

 (after Whitman) ; B-D, of Dicyema 

 typus (after E. van Beneden). Ax 

 axial cell ; K nucleus of the axial 

 cell ; Kz germ cells (from Korschelt 

 and Heider). 



FIG. 196. Infusoriform embryos and their development. A-D, of Dicyema typus ; E-G, of 

 Dicyemella Wagnerii (after van Beneden from Balfour). A-C, developing embryos ; D, embryo 

 from the ventral side, E from the right side, F from the front ; G side view of the urn 

 isolated, gr granular bodies in the urn ; I bed of urn ; u floor of the urn ; r refractive bodies. 



