ENTOZOA. 61 



A small transparent vesicle jutting out from the surface of the 

 germinal membrane is the first vestige of a young hydatid, which 

 speedily becomes opaque in consequence of young cells growing 

 within it. This vesicle very soon separates, and then becomes what 

 Mr. Goodsir terms a secondary hydatid. The young cells which were 

 seen growing within it before its separation now also increase iu size, 

 and soon become parent cells, but do not separate from the germinal 

 membrane of their parent until she escapes from the primitive 

 hydatid. Thus there are four generations, the primitive hydatid 

 still containing the three generations to which she had given birth. 



If the primitive hydatid is buried so deeply in the tissues of the 

 infested being as to prevent the escape of the secondary hydatids, 

 with their two inclosed series of young, decomposition ensues, upon 

 which they speedily disappear. 



And now some may naturally be tempted to ask, having heard this 

 description of a free and independent being, whose tissues are chemi- 

 cally proved to be of an animal nature, imbibing nourishment without 

 vascular connexion with the cavity containing it, and reproducing its 

 kind. How is an animal to be defined, if this be not one ? The 

 answer that the acephalocyst has no mouth, would not be regarded as 

 satisfactory, after the recognition of the animality of the astomatous 

 Polygastria : these, however, are locomotive and can propagate by 

 spontaneous fission. But, definitions apart, our business is to discover 

 to what organic thing the acephalocyst is most similar. 



Almost all the animal tissues result from transformations of free 

 cells, which grow by imbibition, and which develope their like 

 from their nucleus of hyaline. It is to these primitive or fun- 

 damental forms of tissue that the acephalocyst bears the closest 

 analogies in physical, chemical, and vital properties. When the 

 infusorial monads are compared to such cells, and man's frame is 

 said, by a figure of speech, to be made up of monads, the ana- 

 logy is overstrained, because no mere organic cell has its ciUa, its 

 stomachs, its pulsatile sac, &c. So also it appears to me that the 

 analogy has been equally overstrained, which makes the acephalocyst 

 a kind of monad, or analogous species of animal. We may, with some 

 truth, say that the human body is primarily composed or built up 

 of hydatids ; microscopical, indeed, and which, under natural and 

 healthy conditions, are metamorphosed into cartilage, bone, nerve, 

 muscular fibre, &c. When, instead of such change, the organic cells 

 grow to dimensions which make them recognisable to the naked eye, 

 such development of acephalocysts, as they are then called, is com- 

 monly connected in the human subject with an enfeeblement of the 

 controlling plastic force, which, at some of the weaker points of the 



