39 
the cells are so disposed as to indicate four rounded masses. Sections 
of this stage (figs. 56, 57) confirm the presence of the four-rounded 
masses, and in the region of the Y-shaped invagination there is to be 
distinguished another accumulation of cells, the primordium of the 
rostellum. Accumulations of cells were occasionally found in other 
regions of the embryo similar to the four masses mentioned, but since 
65 
Fig. 63. — Longitudinal section of an embryo of H. nana at a later stage than preceding figures: ac., 
suckers; pr, cav., primary caTity; ros., rostellum; sec. car., secondary ca^dty. Enlarged. (After 
Grass! & Eovelli, 1892a, pi. 3, fig. 21.) 
Fig. 64. — Longitudinal section of an embryo of H. nana at about the same stage as the preceding 
figure, through a different plane: ac., suckers; 1. e.xt., external wall; 1. int., internal wall; pr. cav., 
primary cavity; sec. cav., secondary cavity. Enlarged. (After Grass! & Rovelii, 1892a, pi. 3, fig. 22.) 
Fig. 65. — Longitudinal section of an embryo of H. nana at a stage preceding the appearance of the ■ 
definitive hooks; the rostellum is retracted: ac., sucker; hi. p., anterior opening of secondary 
cavity; pr. car., primary cavity; ros., rostellum; sco., scolex; sec. cav., secondary cavity. Enlarged. 
(After Grass! & Rovelii, 1892a, pi. 3, fig. 23. j 
the latter appeared to be constant in many examples, both 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 stage is therefore characterized by the formation of the 
primordia of the rostellum and suckers. 
In the third stage (figs. 59, 60, 62) the anterior part of the body is 
invaginated into the posterior, and the primary cavity has become 
