Under this term we at this day understand nothing beyond those vesicles which we have just described as being inhabited by the echinococcus, but which are in some instances sterile. The above name has been given to this formation in order to designate that supposed independent vitality which the absence of organs still renders problematical. The Acephalo-cyst, which Blainville reckons amongst the "monadaires," and Kuhn compares to Agardh's protococcus, with its multiplication by buds, is in our own opinion not to be held separate from the echinococcus, although the precise relation between the animal and the vesicle is by no means clear.

The relation of the primary acephalo-cyst [the echinococcus-vesicle] to the outer cyst, is analogous to that of a new growth incapsuled by exudation from the surrounding textures.

1. W. Griffith has examined acephalo-cysts and their contents. The transparent fluid, of 1.008 sp. grav., coagulated readily by heat or nitric acid, and contained an inconsiderable amount of fat. A thousand parts yielded fifteen parts of solid ingredients, principally common salt. They left 0.85 per cent, of this salt, a little sulphate of soda, a trace of phosphate of lime, and some albuminous extractive matter, but neither choles-terine nor alkaline phosphates. The envelopes of the hydatids left, when dried, a brown residuum, which dissolved with a deep brown color when boiled with hydrochloric acid, but was not again precipitated on the addition of an alkali. When moist, they dissolved in hydrochloric and in nitric acid, but the solutions were precipitated neither by ferro-cyanide of potassium, nor by tincture of galls. Nothing was dissolved by boiling them in water, for neither by tannic nor by nitric acid was either any precipitate formed, or the fluid gelatinized. When boiled with carbonate of potash, the dried membranes were dissolved with brown coloration, but without any accompanying development of sulphuretted hydrogen, nor any precipitation on the addition of an acid.

Acephalo-cysts, together with the creatures that infest them, are extremely liable to destruction, through hypertrophy, atrophy and consequent perforation of their external coat; but most particularly through inflammation of the latter with its products.

It is not a rare thing to find, within a sac, individual vesicles imperfectly filled, or collapsed, with walls transparent, tumefied, gelatinized, or even degraded to a smeary mass. The contents of such vesicles are turbid. They consist partly of fat-globules with a fine pulverulent point-molecule in great abundance, and the debris of broken up echinococci. Occasionally this conversion affects most, if not all of the vesicles. They burst or rather open out, owing to the increasing tendency to dissolution, until at length the entire contents of the parent cyst are rendered turbid.

The inflammation of the outer sac, a frequent occurrence, is important. It bears the character of inflammation of a sero-fibrous membrane, and throws its products, for the major part, upon the inner surface and into the cavity of the cyst.

It is in many instances to be regarded as a fortunate event, leading as it does to the disruption and extinction of the acephalo-cyst, with its inhabitant animalcules, and in due time to the shrivelling and decay of the entire sac. The contact of the acephalo-cysts with exudate, and the reception of the latter through endosmosis into the walls and cavity of the acephalo-cysts appear to be amongst the most ordinary causes of their dissolution. After the effusion, gradual resorption of a portion of the contents - that is, of the original serous fluid, and of the exudate - follows, whilst another portion thickens to a grayish, unctuous chalk-pap, and eventually cretifies altogether. The sac shrivelling commensurately with the diminution of its contents, becomes obliterated in such wise as ultimately to inclose a mass consisting of variously superimposed residua of acephalo-cysts (echinococcus-vesicles) and of the said chalk-pap or con-crement.

It is not unfrequent for an intense inflammation to terminate in ulceration of the sac, so that an abscess, inclosed within the implicated parenchyma, takes its place. This, together with consecutive suppuration in neighboring textures, may lead to the opening of the sac into another adjoining one; or to its opening externally: or into one or other of the great serous sacs; into the intestinal canal; into the urinary cavities or passages; the gall-ducts, etc. The direction in which such an abscess empties itself decides the question as to the favorable or the unfavorable issue of the case.

The echinococcus and acephalo-cyst are particularly frequent in the liver, less and less so in the subperitoneal, areolar tissue, and in the peritoneum, in the omentum, in the striated muscles, including the heart, in the brain, in the spleen [mostly in concurrence with others in the liver], in the kidneys; very rare in the lungs and bones.

Not unfrequently they occur in several organs simultaneously. Thus they will infest in vast numbers both the peritoneum and the abdominal viscera. In magnitude the sacs may attain, or even exceed the diameter of a foot.

The echinococcus-cysts may become perilous through their volume; and, when present in great numbers, prove fatal through exhaustion and general wasting, as also through the aforesaid inflammatory and suppurative processes.