ANIMAL PAEASITES. 49 



by no means effected right and left in all the hooks in the same 

 direction, but that " whilst the two lateral pairs move downwards 

 from the vertex in the lateral plane nearly simultaneously, this 

 movement only takes place somewhat later in the central pair, 

 and then in the median plane." Thus, sometimes one pair of 

 hooks (the central one), appears to rest, and to hold the embryo 

 firmly in its place whilst the other two lateral pairs are in 

 motion, and sometimes the cental pair appears to be active whilst 

 the other two pairs hold the embryo and preserve it from slipping 

 back, which certainly appears to be most in accordance with nature. 

 By its boring movements the little embryo then penetrates with 

 considerable rapidity through the tissues in its vicinity, and conse 

 quently through the walls of the alimentary canal, and especially 

 of the stomach. This act is the same in all embryos, but as 

 soon as the walls of the alimentary canal are penetrated the case 

 becomes different. The embryos which effect their further 

 development in cold-blooded animals, may probably arrive at 

 once and directly by their boring activity (active migration), at 

 the place where they propose to take up their abode, in order to 

 pass there into the next higher stage of development. But those 

 embryos which are to effect this in warm-blooded animals present 

 a complication of conditions. First of all the migration, as in 

 the former, is an active one, but it must soon become passive, 

 although probably, in most cases, only for a time. Leuckart's 

 careful investigations certainly appear to contradict the assump- 

 tion made by Haubner and myself as regards the greater part of 

 the embryos which subsequently take up their abode in the 

 abdominal cavity and the organs there situated; namely, that the 

 migration of most of the embryos of Tcenice takes place through 

 the ductus choledochus. Leuckart never succeeded in finding 

 free embryos in this canal. It may perhaps even be disagree- 

 able to them, because in it they would have to struggle against a 

 stream of bile flowing down, and not only to bore, which, accord- 

 ing to the previous observations, they like to do, but also by 

 alternately adhering and letting go their hold, to move along by 

 creeping. I now admit that what appeared to me to be the 

 easiest, as it offered an open way to the embryos, would rather 

 be very difficult to them. The passage by the ductus chole- 

 dochus, therefore, can be selected by them, if ever, only in very 

 rare, exceptional cases. I believe also, that (as these animals do 

 not appear to be particularly fond of creeping without boring) 



