*The Discoveries of Herbst.*--There are but few points in the doctrine of the external means or conditions of organogenesis which have a more special bearing on the specification of proper form, and which therefore require to be described here a little more fully. All these researches, which have been carried out almost exclusively by Herbst,[37] relate to the effect of the chemical components of sea-water upon the development of the sea-urchin. If we select the most important of Herbst’s results, we must in the first place say a few words on the part taken by lime or calcium, not only in establishing specific features of form, but in rendering individual morphogenesis possible at all. Herbst has found that in sea-water which is deprived of calcium the cleavage cells and many tissue cells also completely lose contact with each other: cleavage goes on quite well, but after each single division the elements are separated; at the end of the process you find the 808 cells of the germ together at the bottom of the dish, all swimming about like infusoria. There seems to be some influence of the calcium salts upon the physical state of the surfaces of the blastomeres. [37] *Arch. Entw. Mech.* 17, 1904. It is not without interest to note that this discovery has an important bearing on the technical side of all experiments dealing with the isolation of blastomeres. Since the separation of the single cleavage elements ceases as soon as the germs are brought back from the mixture without lime into normal sea-water, it of course is possible to separate them up to any stage which it is desired to study, and to keep them together afterwards. Thus, if for instance you want to study the development of isolated cells of the eight-cell stage, you will leave the egg in the artificial mixture containing no calcium until the third cleavage, which leads from the four- to the eight-cell stage, is finished. The single eight cells brought back to normal sea-water at this point will give you the eight embryos you want. All researches upon the development of isolated blastomeres since the time of Herbst’s discovery have been carried out by this method, and it would have been quite impossible by the old method of shaking to pursue the study into such minute detail as actually has been done. It may be added that calcium, besides its cell-uniting action, is also of primary importance in the formation of the skeleton.