DEVELOPMENT OP EMBRYO. 77 



stantly repeated in other species of animals, in a form resembling 

 that in which we are acquainted with the platycercal and acercal 

 Cestodea of the cold-blooded animals ; or, for example, if we 

 meet with Cysticercus cellulose in some animal in a form similar 

 to that of Stein's cestoid worm of Tenebrio molitor, or that of Von 

 Siebold's worm of Arion. Until this is the case we cannot place 

 the cysticercal Cestodea in opposition to the other only cysti- 

 cercoid forms in the relation of normality and degeneration. 



In conclusion, we have still to refer here to the circumstance 

 that a great number of the cestoid embryos which become 

 developed to the scolex state, die and are desti'oyed, either by a 

 natural or a pathological death. Of a natural death of the 

 cystic worms caused by the weakness of age, Ave could only 

 speak if we were acquainted with the extreme limits of the 

 existence of these Cestodea, which is not the case at present. 

 We only know that their life in the c} r stic state may pro- 

 bably extend to several years. They die a pathological death 

 when primary or secondary inflammatory processes and altered 

 conditions of secretion are set up in the enveloping cyst (in 

 consequence of inflammatory actions in the organ inhabited by 

 the worm). It may easily be that previous phenomena of 

 the same kind, but of a lower degree, occurring during the 

 development of the embryonal vesicle into a scolex may lead 

 to the formation of acephalocysts, and that the similar phe- 

 nomena of a higher degree occurring subsequently, after the 

 development of the scolex, may not merely arrest, but actually 

 destroy, the life of the individual. In such cases Ave see the 

 enveloping cysts thickened with layers of exudation, the vessels 

 increased in number and size, and beset on their inner surface 

 Avith small red excrescences (such as are also found elseAvhere in 

 serous cavities), amongst which calcareous masses and masses of 

 cholesterine are sometimes deposited. Their fluid contents are 

 no longer limpid and thin, but bloody, dingy yelloAV (from 

 decomposition of the colouring matter of the blood), and more 

 tenacious. The walls of the cystic worm also become turbid, 

 yelloAV, dingy, and opaque. Up to this time the phenomena 

 are the same in all vesicular Avorms, but subsequently differences 

 occur in the details, according to the species to which these 

 animals belong. Those Avhich are knoAvn under the names of 

 Coenurus and Cysticercus, from the moment Avhen the nutritive 

 fluid contained in them acquires the nature just described, allow 



