BY PROFESSOR W. A, HASWELL. 507 



form nearly related to that species."^ In the first point it also 

 resembles D. acanthocephalam and other members of Rudolphi's 

 sub-genus Echinostomum. 



Development of the Cercari^. 



For what we know of the development of the Cercaria within 

 the Sporocyst or Redia we are mainly indebted to Leuckart (17), 

 Thomas (14), Heckert, Schwarze (12), and Loos (9)- In spite of 

 their labours, however, it must be said that a good many points 

 remain obscure. This holds good particularly with regard to the 

 precise mode of origin of certain of the organs; and it is due 

 mainly to the almost complete absence of differentiation among 

 the cells (meristem cells of Schwarze and others) of the earl}- 

 stages, the rudiments of various parts having the appearance of 

 becoming quite suddenly crj^stallised out from a previousl}^ homo- 

 geneous mass. 



As already stated, the embryos of the Cercaria from Mytilus 

 latns are set free from the ovary when they have attained a 

 diameter of about 0*025 mm.* At about this stage (figs. 15 and 16) 

 the embryonic investment is formed. First a single cell on the 

 surface becomes flattened out, then others become similarl}^ 

 modified until the spherical mass of cells becomes enclosed in a 



* I infer this from Braun's reproduction (3, taf. xxi., fig, 8) of a figure by 

 Stossich, the original paper by that author (in the Bolletino della Soc. adriat. 

 scienze natur, Trieste, vol ix., 1885) not being accessible to me. 



* Reuss (11) met frequently with a stage in which one large cell (ovum) 

 was accompanied by three considerably smaller cells, with nuclei 0-0025 in 

 diameter with uniformly distributed, coarsely granular chromatin. Since the 

 large cell next divides into quite equal and similar cells he takes these 

 previously formed three smaller cells to be of the nature of polar bodies, and 

 their formation to be a maturation process. This stage I have not observed. 

 But in a large proportion of specimens there occur lying loose in the Sporo- 

 cyst in the immediate neighbourhood of the ovary a varying number of cells 

 (fig. 14) which have homogeneous deeply staining nuclei 0*002 mm, in 

 diameter. If these are not of the nature of polar bodies it seems difficult 

 to account for them. 



