t in all directions。 As a rule； the centre of the cell contains one or more different pigments which under the influence of nerves can spread out separately or together into the ramifications。 These phenomena of spreading and retraction of the pigments into or from the ramifications of the pigment…cells form on the whole the basis for the colour changes under the influence of environment。 Thus Keeble and Gamble observed that Macromysis flexuosa appears transparent and colourless or grey on sandy ground。 On a dark ground their colour becomes darker。 These animals have two pigments in their chromatophores； a brown pigment and a whitish or yellow pigment； the former is much more plentiful than the latter。 When the animal appears transparent all the pigment is contained in the centre of the cells； while the ramifications are free from pigment。 When the animal appears brown both pigments are spread out into the ramifications。 In the condition of maximal spreading the animals appear black。
This is a comparatively simple case。 Much more complicated conditions were found by Keeble and Gamble in other crustaceans； e。g。 in Hippolyte cranchii； but the influence of the surroundings upon the colouration of this form was also satisfactorily analysed by these authors。
While many animals show transitory changes in colour under the influence of their surroundings； in a few cases permanent changes can be produced。 The best examples of this are those which were observed by Poulton in the chrysalids of various butterflies； especially the small tortoise…shell。 These experiments are so well known that a short reference to them will suffice。 Poulton （Poulton； E。B。； 〃Colours of Animals〃 （The International Scientific Series）； London； 1890； page 121。） found that in gilt or white surroundings the pupae became light coloured and there was often an immense development of the golden spots； 〃so that in many cases the whole surface of the pupae glittered with an apparent metallic lustre。 So remarkable was the appearance that a physicist to whom I showed the chrysalids； suggested that I had played a trick and had covered them with goldleaf。〃 When black surroundings were used 〃the pupae were as a rule extremely dark； with only the smallest trace； and often no trace at all； of the golden spots which are so conspicuous in the lighter form。〃 The susceptibility of the animal to this influence of its surroundings was found to be greatest during a definite period when the caterpillar undergoes the metamorphosis into the chrysalis stage。 As far as the writer is aware； no physico…chemical explanation； except possibly Wiener's suggestion of colour…photography by mechanical colour adaptation； has ever been offered for the results of the type of those observed by Poulton。
V。 EFFECTS OF GRAVITATION。
（a） EXPERIMENTS ON THE EGG OF THE FROG。
Gravitation can only indirectly affect life…phenomena； namely； when we have in a cell two different non…miscible liquids （or a liquid and a solid） of different specific gravity； so that a change in the position of the cell or the organ may give results which can be traced to a change in the position of the two substances。 This is very nicely illustrated by the frog's egg； which has two layers of very viscous protoplasm one of which is black and one white。 The dark one occupies normally the upper position in the egg and may therefore be assumed to possess a smaller specific gravity than the white substance。 When the egg is turned with the white pole upwards a tendency of the white protoplasm to flow down again manifests itself。 It is； however； possible to prevent or retard this rotation of the highly viscous protoplasm； by compressing the eggs between horizontal glass plates。 Such compression experiments may lead to rather interesting results； as O。 Schultze first pointed out。 Pflueger had already shown that the first plane of division in a fertilised frog's egg is vertical and Roux established the fact that the first plane of division is identical with the plane of symmetry of the later embryo。 Schultze found that if the frog's egg is turned upside down at the time of its first division and kept in this abnormal position； through compression between two glass plates for about 20 hours； a small number of eggs may give rise to twins。 It is possible； in this case； that the tendency of the black part of the egg to rotate upwards along the surface of the egg leads to a separation of its first cells； such a separation leading to the formation of twins。
T。H。 Morgan made an interesting additional observation。 He destroyed one half of the egg after the first segmentation and found that the half which remained alive gave rise to only one half of an embryo； thus confirming an older observation of Roux。 When； however； Morgan put the egg upside down after the destruction of one of the first two cells； and compressed the eggs between two glass plates； the surviving half of the egg gave rise to a perfect embryo of half size （and not to a half embryo of normal size as before。） Obviously in this case the tendency of the protoplasm to flow back to its normal position was partially successful and led to a partial or complete separation of the living from the dead half； whereby the former was enabled to form a whole embryo； which； of course； possessed only half the size of an embryo originating from a whole egg。
（b） EXPERIMENTS ON HYDROIDS。
A striking influence of gravitation can be observed in a hydroid； Antennularia antennina； from the bay of Naples。 This hydroid consists of a long straight main stem which grows vertically upwards and which has at regular intervals very fine and short bristle…like lateral branches； on the upper side of which the polyps grow。 The main stem is negatively geotropic； i。e。 its apex continues to grow vertically upwards when we put it obliquely into the aquarium； while the roots grow vertically downwards。 The writer observed that when the stem is put horizontally into the water the short lateral branches on the lower side give rise to an altogether different kind of organ； namely； to roots； and these roots grow indefinitely in length and attach themselves to solid bodies； while if the stem had remained in its normal position no further growth would have occurred in the lateral branches。 From the upper side of the horizontal stem new stems grow out； mostly directly from the original stem； occasionally also from the short lateral branches。 It is thus possible to force upon this hydroid an arrangement of organs which is altogether different from the hereditary arrangement。 The writer had called the change in the hereditary arrangement of organs or the transformation of organs by external forces HETEROMORPHOSIS。 We cannot now go any further into this subject； which should； however； prove of interest in relation to the problem of heredity。
If it is correct to apply inferences drawn from the observation on the frog's egg to the behaviour of Antennularia； one might conclude that the cells of Antennularia also contain non…miscible substances of different specific gravity； and that wherever the specifically lighter substance comes in contact with the sea…water （or gets near the surface of the cell） the growth of a stem is favoured； while contact with the sea…water of the specifically heavier of the substances； will favour the formation of roots。
VI。 THE EXPERIMENTAL CONTROL OF ANIMAL INSTINCTS。
（a） EXPERIMENTS ON THE MECHANISM OF HELIOTROPIC REACTIONS IN ANIMALS。
Since the instinctive reactions of animals are as hereditary as their morphological character； a discussion of experiments on the physico… chemical character of the instinctive reactions of animals should not be entirely omitted from this sketch。 It is obvious that such experiments must begin with the simplest type of instincts； if they are expected to lead to any results； and it is also obvious that only such animals must be selected for this purpose； the reactions of which are not complicated by associative memory； or； as it may preferably be termed； associative hysteresis。
The simplest type of instincts is represented by the purposeful motions of animals to or from a source of energy； e。g。 light； and it is with some of these that we intend to deal here。 When we expose winged aphides （after they have flown away from the plant）； or young caterpillars of Porthesia chrysorrhoea （when they are aroused from their winter sleep） or marine or freshwater copepods and many other animals； to diffused daylight falling in from a window； we notice a tendency among these animals to move towards the source of light。 If the animals are naturally sensitive； or if they are rendered sensitive through the agencies which we shall mention later； and if the light is strong enough； they move towards the source of light in as straight a line as the imperfections and peculiarities of their locomotor apparatus will permit。 It is also obvious that we are here dealing with a forced reaction in which the animals have no more choice in the direction of their motion than have the iron filings in their arrangement in a magnetic field。 This can be proved very nicely in the case of starving caterpillars of Porthesia。 The writer put such cater