352 



THE DEVELOPMENT OF CESTODES. 



are gradually becoming mature (for it is only in the bookless Tmnia 

 saginata that tbe original state persists), this ridge grows further and 

 further over the rostellum, until (Fig. 189) its margins coalesce in the 

 centre, and form tbe layer in which tbe posterior ^ processes of the 

 books are embedded (see Fig. 151). The metamorphosis of the hooks 

 themselves has been already described (p. 287). They appear first 

 as soft, thin, little cones, which grow into the cavity of the head with 

 their points upwards and their concave sides turned outwards. Be- 

 fore they become elevated, one finds countless fine points on the 

 periphery of the subsequent rostellum, some of which grow directly 

 into the cones, while the greater number of them speedily disappear. 



Fig. 197.— Head and body of 

 Ccermrus, in situ. { x 100.) 



Fig. 19S.—Cysti- 

 cercus pisiformis with 

 head half evaginated. 

 (X 6.) 



Fig. 199.— Head and 

 body of C>/sticercu$ 

 pisiformis in com- 

 pletely evaginated state, 

 (X 19.) 



The histological differentiation is in the majority of cases complete 

 towards tbe end of the second month. The head has then essentially 

 its adult organisation,^ and almost its adult size, although it still 



^ The " posterior process of the hook " is the one directed towards the apex of the 

 head.— W. E. H. 



' I may briefly note that one can already recognise under the rostellum the " plate-like 

 muscles described by Nitsche (Zeitschr. f. wiss. Zook, Bd. xxiii., p. 181, 1880), and m 

 euocessful sections the head ganglia. The latter are widely separated from one another by 

 the presBure which acta longitudinally on the head, so that they have the greater part of 

 the rostellum lying between them. 



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