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" experimental embryology " in the narrow sense, where germ- cells and developing ova are subjected to artificial stimuli, con- straints, and disturbances, but also as regards later stages of embryonic life. Here might also be considered the observations of Carrel and others on the prolonged life of pieces of tissue in artificial cultures, and the important researches of Ross Harrison on the growth of a nerve fibre. It is of zoological as well as physiological interest that this growth should be closely comparable to the protrusion of a pseudopodium by an amoeba.

Experimental Embryology. In illustration of what may be included under the term " experimental embryology," we may take Godlewski's remarkable cases of the fertilization of an ovum with the spermatozoon of an unrelated animal. A frag- ment of the ovum of Echinus, bereft of a nucleus, was fertilized by the spermatozoon of the Crinoid Antedon. The segmentation, the gastrulation, and the formation of the mesenchyme followed the type of the maternal parent, which seems to show that the cytoplasm may count for much in inheritance. The ova of Sphaerechinus were fertilized by the spermatozoa of the Annelid Chaelopterus, and the union of the two nuclei was observed. But before the nucleus divided, the chromatin from the sperma- tozoon was ejected, as if the incompatibility was too pronounced. In another set of experiments Godlewski altered the composition of the sea water containing sea-urchin ova, and then introduced the spermatozoa of Chaetopterus or of the mollusc Denlalium. The result was development, which was followed to the Pluteus stage. The further experiment was made of adding spermatozoa of the sea-urchin to those of the worm or of the mollusc, with the result that no development occurred. Godlewski speaks of the antagonism of the unrelated spermatozoa, and Bohm (1921, p. 27) calls attention to the possible analogy with the antagonistic action of serums derived from distantly related species. Nothing, he says, is more fruitful in biology than the bringing together of two sets of facts which appear at first sight to belong to very different provinces. Enough has been said to indicate the interest of this relatively young inquiry.

Artificial Parthenogenesis. It was discovered independently by Yves Delage and by Jacques Loeb that ova which do not normally show parthenogenetic or aspermic development may be induced to do so by very varied artificial stimulation. The prosecution of this kind of experiment has made it plain that there are usually two distinct events the first, a stimulation which induces cleavage but is apt to lead to disintegration; the second, a counteracting or corrective influence which steadies develop- ment, acting as a life-saving brake. Thus, to take one of Loeb's methods, if the eggs of the sea-urchin are subjected for a short time to the influence of some fatty acid like butyric acid, cleavage sets in, but it is then necessary to place the ova in hypertonic sea water to keep them on lines of safety. Or, to take one of Delage's methods, the ova may be first activated by a combined application of tannin and ammonia, and then replaced in ordinary sea water. Or, to take Bataillon's method, if the eggs of the frog are first activated by pricking them with a very fine stilet of glass or platinum, and then steadied by washing them with blood, which allows of the entrance of a corpuscle, normal de- velopment may follow, and several young frogs, of both sexes, have been reared.

Experimental Teratology. A good instance of the modern endeavour to supplement the study of form by the study of function, morphology by physiology, may be found in the de- velopment of experimental teratology. Accurate descriptions of monstrosities and abnormalities are valuable, but their significance is increased when some light can be thrown on how the peculiarities in question came about. It has been shown that defective nutrition of the embryo, especially at critical stages, may lead to " arrests of development," such as harelip in Man. It has been shown that conditions which bring. about local slowing or quickening of the rate of development may result in abnormal asymmetry, coalescence, separation, and the like. But a more precise illustration may be useful. Dr. E. I. Werber (1916) subjected the developing eggs of the American minnow, Fundulus, to various reagents, such as butyric acid, which

brought about numerous monstrosities in eyes and ears, nostrils and mouth, heart and fins. The chemical intrusion seemed to dislocate and partially dissolve the germinal material, especially toward the head end of the embryo. As one of the results of a disturbance of carbohydrate metabolism is the production of butyric acid, the theory almost suggests itself that a poisoning of a mammalian mother's constitution with butyric acid might account for histolytic monstrosities in the offspring. Werber has also shown that a breaking-up (blastolysis) of the optic club brings about the formation of many lenses. The optic club fragments exert on various parts of the skin which would not naturally form a lens a specific effect, like that of a ferment, which induces multiple lens-formation.

Sex Determination. Without encroaching on the article SEX, we may notice the general fact that the inquiry has become wholesomely experimental along several distinct lines, (a) The striking work of Geoffrey Smith and Potts, following Giard's pioneer observations, has shown that the castration of a male crab by a Rhizocephelan parasite induces an expression of latent feminine and female features. The male put on abdominal limbs like those of a female, and the gonad showed ova; and Smith's theory, stated briefly, was that the parasite alters the composition of the male's blood to or toward a female condition, and that this is naturally followed by the development of latent female secondary characters, or by the regeneration of an ovary instead of a testis from the indifferent residue that remains at the end of the infection. Metabolism-stimulation activates latent sex characters. Using the terminology of The Evolution of Sex (Geddes and Thomson, 1889), Geoffrey Smith summed up: " This adaptive regulation consists in the production of at least a partially female condition of anabolism as opposed to a wholly male condition, the female condition being preponderantly anabolic or conservative, as opposed to the katabolic male condition, and, by this change from a katabolic to a more anabolic condition, the animal can withstand better the drain on its system increased by the parasite." (b) Many experiments show that latent masculine characters in a female animal, or latent feminine characters in a male, may be activated by changes in the internal secretions of the gonads. (c) Baltzer (1914) has shown that if the very young sexually indifferent larvae of the worm Bonel- lia, just hatched from the eggs, happen to become attached to the proboscis of an adult female, they develop into males; whereas, if they fail to attach themselves and sink into the sand or mud, they develop slowly (almost exclusively) into females. But the story does not end here. Baltzer helped some of the very young free-swimming Bonellias to attach themselves to the proboscis of a full-grown female; those that he left attached for a very short time developed into almost perfect females; those that he left attached for a long time became perfect males, if such degenerate pigmies can be called perfect; while those that he left for intermediate periods showed practically all grades of inter-sex. These fine experiments point to a conception of sex as plastic, reversible, constitutional, and quantitative, (d) The recent work of Oscar Riddle has made it practically certain that pigeons produce two kinds of eggs which differ in the rate or intensity of their chemical processes. One kind of egg has a low storage capacity, a high oxidizing capacity, and a relatively higher intensity of metabolism ; and such a type of egg develops into a male bird. The contrasted type of egg develops into a female bird, while an intermediate type of egg, produced for instance early in the season, will develop into a female bird with an admixture of masculine features. Femaleness in an egg is associated with high storage capacity, less intensity of meta- bolism, lower percentage of water, higher total of fat and phos- phorus, and greater energy value in the yolk as determined by the calorimeter. It is of further interest to notice that analyses of the blood of adult cocks and hens show a persistence of con- stitutional differences analogous to those which mark the two kinds of ova. These experiments furnish a corroboration of the thesis, suggested in 1889 by Geddes and Thomson, that female- ness is associated with a relative preponderance of constructive up-building, or anabolic processes, and conversely for maleness.