T^NIAD^. Ill 



due to the active migrations and growth of the six-hooked embryos, 

 and in the earlier stages Leuckart has found distinct evidences of 

 the existence of the six-hooked boring apparatus within the con- 

 tents of the vesicles. Stein has also observed them from another 

 species of tapeworm. It was not to be wondered at that 

 difficulties would be encountered in determining with certainty 

 the connection between the six-hooked haBmatozoa and the hver 

 vesicles, because, very soon or immediately after the passage of 

 the former (embryos) out of the blood into the liver-parenchyma, 

 they appear to commence the metamorphosis by which the six- 

 hooked embryo is transformed into a true cysticercus. The liver 

 vesicles, in their later stages of growth, are, in pomt of fact, young 

 cysticerci enclosed in capsules of connective tissue, formed by the 

 liver, as it were, in self-defence. The six-hooked embryos, whilst 

 still in the blood, may be said to wander "passively ;" but, in the 

 first instance, they must have passed "actively" into the vessels 

 of the intestinal villi after escaping from their egg-coverings. 

 Carried by the circulating current into different parts of the body, 

 they again appear to exercise their boring propensities in that 

 particular and suitable locality (liver) which a certain appetite, 

 so to speak, instinctively induces them to select. 



Before tracing onward their further progress, I may revert to 

 the state of things observable at the third week. At this period 

 the partially metamorphosed embryos are about one line in length, 

 and they have acquired a structureless epidermis, in addition to 

 two special sub-dermal layers of muscular fibre. At this time also 

 there arises, from the development of nucleated cells, a turbid 

 appearance at the anterior end of the embryo. This condensation 

 of the parenchyma is destined to form the head of the so-called 

 Cysticercus pisiformis. A folding inwards of the epidermis at the 

 anterior end of the embryo forms what is called the " cephahc 

 pit;" an opening being thus left at the centre of the point of 

 inversion. Subsequently, the little embryo displays more marked 

 differentiations of structure, and the turbid mass shows two 



