Page:Popular Science Monthly Volume 58.djvu/452

444 of Loeb that the eggs of the sea-urchin may be caused to develop, without the influence of the male element, by treatment with solutions of magnesium chloride or other substances added to the sea-water. Wilson has now examined the internal processes occurring in these eggs. Phenomena of this character had been earlier studied by Richard Hertwig and Morgan, whose work paved the way for that of Loeb; but neither of these observers succeeded in obtaining complete embryos, the eggs only having passed through the initial stages of development. Wilson's observations bring the decisive proof that the eggs, developed under these conditions, have not been accidentally fertilized. It is well known that in the fertilization of the egg an equal number of chromosomes are contributed by the egg and the spermatozoon, this number being in every known case one-half that characteristic of the tissue cells of the species. If, therefore, the magnesium eggs really develop without union with a spermatozoon, we should expect to find them showing but one-half the number of chromosomes occurring in fertilized eggs. Such is, in fact, the case in the magnesium eggs (of Toxopneustes), the number of chromosomes being here 18, while in normal fertilization it is 18 plus 18, or 36. Every doubt is thus removed regarding the accuracy of Loeb's general result. Interesting light is thrown by the observations on many features of the process of normal fertilization. According to Boveri's well-known theory, the egg is induced to develop through the importation of a centrosome carried by the spermatozoon. In the magnesium eggs this is obviously out of the question; and Wilson's studios, supplementing the earlier ones of Hertwig and Morgan along the same lines, give strong evidence not only that the importation of a centrosome is not necessary to development, but also that the centrosomes of the dividing magnesium eggs are formed de novo out of the egg-substance. As observed by Morgan, these eggs often become filled with large numbers of asters, each of which contain a centrosome. One of the most interesting results of Wilson's work is the discovery that these asters may multiply by division and form centers of cytoplasmic division, even when they have no connection with nuclear material. The important point was determined also that similar asters and centrosomes, likewise capable of division, are formed in non-nucleated egg-fragments obtained by shaking the eggs to pieces—a fact which shows that the formation of a centrosome may be wholly independent of the nucleus.

a second paper Wilson described experiments on etherizing normally fertilized eggs at various stages, the results of which bear nearly on some of the questions suggested by the magnesium eggs. The principal result of these experiments was to show that division of the nucleus and that of the cell-body, though parallel, are in considerable measure independent processes, which is in accordance with earlier studies by Hertwig, Demoor and others. The results give, further, considerable ground for the conclusion that the rays of the radiating systems or asters in dividing cells cannot be regarded as fixed, fibrillar structures, as is assumed by most of the prevailing views, but are tracts of protoplasmic flow, as was many years ago maintained by Fol and Bütschli. It was shown also that by suitable etherization of the eggs and subsequent transfer to sea-water, the type of fertilization characteristic of the sea-urchin may be artificially changed into that normally occurring in the starfish, and in many worms and mollusks; and, in like manner, that the cleavage of the egg may be transformed into a mode that is typical of many of the cœlenterates and arthropods. These observations show that many new and interesting conclusions bearing on the early stages of development may be looked for by further experimental studies along the lines marked out fourteen years ago by O. and R. Hertwig,