the cells are so disposed as to indicate four rounded masses. Sections 
of this stage (hgs. 56. 57) conhrni the presence of the four-rounded 
masses, and in the region of the Y-shaped invagmation there is to he 
distinguished another accumulation of cells, the primordium of the 
rostellum. Accumulations of cells were occasionally found in other 
regions of the emhryo similar to the four masses mentioned, hut since 
ac: 
■sec.csY. 1 
63 
Fig. 63. — Longitudinal section of an embryo of H. nana at a later stage than preceding tigures; ac.. 
suckers; pr. cav., primary cavity; I'os.. rostellum; sec. cav.. secondary cavity. Enlarged. After 
Grass! A Eovelli, lS92a. pi. 3. fig. 21. ) 
Fig. 64.— Longitudinal sec-tion of an embryo of H. nana at about the same stage as the preceding 
figure, through a different plane; ac.. suckers; L e.rt.. external wall; L int.. internal wall; pr. cav.. 
primary cavity; sec. cav.. secondary cavity. Enlarged. After Grassi A Rovelii, lS92a, pi. 3. fig. 22.) 
Fig. 65. — Longritudinal secrion of an embryo of H. nana at a stage preceding the appearance of the 
definitive hooks; the rostellum i< retracted; ac.. sucker; hi. p.. anterior opening of secondary 
cavity; p?-. car., primary ca\dty; ra<?.. rostellum; sco.. scolex; sec. car., secondary cavity. Enlarged. 
(After Grassi A Rovelii. 1892a, pi. 3. fig. 23. 
the latter appeared to he constant in many examples, hoth in the fresh 
state and when sectioned, the conclusion was arrived at that the four 
masses in question were the primordia of the suckers. 
The second staofe is therefore characterized liv the formation of the 
primordia of the rostellum and suckers. 
In the third stage (tigs. 59. 60. 6iq the anterior part of the body is 
invaginated into the posterior, and the primary cavity has become 
