The behaviour of the embryo under the action of reagents of various kinds is remarkable. Thus, when on the 5th of Sept., 1870, I placed some ova in brackish water, of the strength of two parts of fresh water to one of pure sea-water, their contents were readily developed, though the escaping embryos did not swim vigorously. When again I placed some other eggs in pure sea-water, their contained embryos became instantly transfixed, the vibratile cilia of the head being rigid and motionless. At first I naturally concluded that the embryos were killed outright; but, to my great surprise, the shock passed away in about half an hour, when they revived and were soon afterwards hatched. One of the larvæ thus set free carried off several of the loose intra-chorional globules which had, during the period of transfixion, become firmly adherent to the ends of the caudal cilia. Here I may remark upon a decided difference observable between the cilia of the head and body respectively. The former are at all times vibratile, active, and conspicuous, whilst the latter are more delicate, capable of comparatively little motion, and partaking more of the character of fine setæ. In length their general measurement varies from 1/2500″ to 1/2000″. The action of pure sea-water on the free animalcules, previously immersed in fresh or brackish water, was equally striking. All, without exception, immediately became paralysed and almost motionless; nevertheless, on again adding fresh water, several entirely recovered. It is worthy of notice that in these cases the cephalic cilia furnished the first indications of returning viability. I was particularly struck with the behaviour of one embryo, which, under the stimulus of the sudden shock, retracted its cone-shaped head almost entirely within the general cavity of the body (fig. 14, lower specimen). In their moribund condition, whatever shape the embryo retained, the sarcodic contents gradually faded away; the outline of the creature, however, becoming more marked (fig. 16). Usually the body of the animalcule became elongated whilst expiring in sea-water. Under other circumstances the embryo frequently bursts; the sarcodic contents escaping in the form of amœba-like bodies and the cilia retaining their powers of movement long after all traces of the sarcode have disappeared. [Illustration: FIG. 17.--Ciliated embryo of _Fasciola hepatica_, showing the so-called eye-spot. After Leuckart.]