B.EGELATION. 798 KEGENEBATION. is generally covered ^vith a tliin film of ice for the reason that the snow melted by the pressure of the wheels freezes as soon as the pressure is removed. A snowball is made by the pressure of the liands causing the snow to melt, and then the water is solidified. Consequently if the snow is dry and cold and below the freezing point, the pressure of the hands will not suffice. If a press is used, the snow will be melted and cylindrical or other forms of transparent ice can be formed. The well-known experiment of looping a cord or wire around a block of ice and attaching a ueight will also sliow regelation. Here the pressure on the cord melts the ice and allows the string to cut its way tlirough the block, but at the same time the water thus formed is again frozen and the block left in its original solid condition. A union between two pieces of ice will take place when they are in contact under water, even if the temperature is considerably above that of the air. In such a case, however, the capillary action of the film of water between the two faces renders the internal pressure less than the external and acts to bring the two pieces together with pres- sure. The phenomenon of regelation is also quoted to explain the formation and movement of glaciers ( q.v. ) . The glacier in its progressive movement acts much as a viscous solid, the top moving faster than the bottom and the middle faster than the sides. The pressure of the vast quantity of snow above melts the ice or snow at the bottom, and this, escaping and flowing down, freezes and solidifies, a gradual slipping away of the base occurring. As the foot of the glacier descends it reaches warmer regions, so that melt- ing will take place with less pressure and the water will drain off'. In this way it is possible to explain much of the formation and movement of glaciers, though of course the problem is very complex and other causes exert powerful influ- ences. Consult Preston, Theory of Heat (New York, 1894). See Heat. REGENERATION {'La.t. ' regeneratio, from regciicrarc, to generate anew, from re-, back again, anew -|- generare, to beget, from genus, family). Replacement of lost parts, renewal of organs, or comjjletion of an organism from a part. In 1744 the Swiss naturalist Trembley foimd that on cutting hydras in two, or slicing them into thin rings, from each ring grew out a crown of tentacles; and in splitting them into longitudinal strips each portion became a well- shaped hydra. Finally he turned one inside out and in a few days the evaginated hydra swallowed pieces of meat, though its former stomach-lining had now become its skin. Bonnet found that from the same region of a worm, like the earth- worm, a head or tail may arise according to whether that region happens to lie at the anterior or posterior end of the cut surface. Thus if a worm ( Lumbriculus ) be cut into two pieces, a new tail will develop from the posterior end of the anterior piece, and a new head from the front end of the posterior piece. In another species of fresh-water annelid Bonnet found that a new tail developed at the anterior end of the posterior piece, and not a head. As the result of recent experiments on the earthworm it is ascertained that if from one to five of the anterior segments be cut olT, the same number come back ; if more are cut off, the process of regeneration begins only after a longer interval, and only four or five segments come back as a rule; if the cut be beliind the middle, the time before regeneration begins is still longer, and fewer worms succeed in regenerating at all. Each end of the body can regenerate in its own direction onlv. BEGENEBATION OF HYDEA. 1, Two anterior pieces of hydra uuited by their arborai ends; 2. hydra split in two. hanging: vertically downward ; 3. two posterior ends o( hydra united by oral surfaces: 4, five pieces united as shown by arrows. (After Morgan.) The effect produced by external factors in experiments on regeneration are noteworthy. A hydroid (Eudendriuni) failed to develop new heads when kept in the dark, but when placed in the light the new heads quickly appeared, show- ing that light acts as a stimulus; and instead of a liead a root will develop at the distal end of a piece if that end be biought into contact with some fixed object, and conversely a new head will appear at either the nearest or farthest end if the end be freely surrounded with water ; in this case the external agent is the stimulus which determines the difl'erentiation of the part; and experiments have produced some extremely curious manifestations of how this stimulus acts. For example, a piece cut from the stem of Anten- nularia and suspended vertically in the water will develop a new stem at the upper end, and roots at the lower end, regardless of the normal position. Here gravity alone determines that the upper end shall grow into a new stem and the lower end into a new root. In the one case, where the upper end corresponds to the proximal end of the original hydroid, the new part (a stem) replaces the lost root-end, and this change Loeb calls 'heteromorphosis.' In the other case, says ilorgan, where the upper end corresjjonds to the distal end of the original hydroid, the new part (a stem) replaces the lost part of the stem. This is what is usually meant by 'regeneration.' It is of some theoretical interest to know whether the old cells directly form the new tissue or whether reserve cells are present that bring about the result. From experiments thus far made Morgan thinks that this may vary with difl'erent forms. In Planaria, a flatworm. a new head forms on the anterior end of the posterior half of the worm, but the entire anterior half is never replaced by new tissue. In this same plana- rian Randolph has discovered a most important relation existing betweeu the old and new parts. If a planarian be cut in two longitudinally in the median plane, the right half regenerates a new left half of the same size as the part removed, and the left half also develops a new right half
