326 



THE ANATOMY OF CESTODES. 



occupy the largest space in the egg (0'03 mm.), and are surrounded 

 by a fine granular substance, in which some coarser shining grains of 

 a fatty nature are generally observed. But afterwards the yolk-cells 

 alter, larger and smaller ones (O007 mm.) can be distinguished, and as 

 the former usually occur in threes, the latter probably originate from one 

 of the four previous balls. And further, it is only those smaller cells 

 which produce the body of the embryo. They multiply without losing 

 perceptibly in size, and gradually collect into a spherical cluster near the 

 large cells, but this mass is larger (0'02 mm.) than the one formerly found 

 within the pale shell-membrane. The vesicular nucleus has a diameter 

 of O08-O1 mm., and a distinctly defined nucleolus. In some cases four 

 or five cells are found instead of three, and it appears as though one of 



FIG. 176. Emlryonic development of Ttrnia scrrata (A ), and T. marginata (B) ; a, eggs 

 before segmentation. ( x 550. ) 



the smaller cells sometimes detached itself from the rest and formed 

 a covering-cell. We can hardly doubt that these larger cells, in spite 

 of their peculiar origin and smaller numbers, are comparable to the 

 already mentioned cell-sheath of other species, for, like it, they do not 

 take the slightest part in the formation of the embryo, but are per- 

 manently present outside it in the surrounding shell. The latter 

 originates in the form of a very delicate and smooth cuticle, as soon 

 as the embryo has attained a size of about 0'02o mm... and has ex- 

 changed the former cellular structure for an almost homogeneous 

 appearance. The rod-like fringe only makes its appearance gradually 

 by the growth of protuberances upon the external surface. As the 



