Driesch (1900) reëxamined this point, and found that the embryos from isolated blastomeres may use the proportionate number of cells. I have made a new study of the problem on a larger scale and have found that my earlier statement, as well as that of Driesch, is substantially correct, and that the difference that we found is due to the time at which the embryos gastrulate. Thus the one-half embryos and even the one-fourth embryos, that gastrulate as soon as (or only a little later than) the normal, whole embryos, turn into the archenteron about one-half and one-fourth the number of cells invaginated in the whole embryo; but those partial embryos that gastrulate later (as most of them do) turn into the archenteron more than a half or a fourth of the number of cells turned in _at first_ by the whole embryo. This difference between the early and the retarded partial embryos is in large part due to a slow increase of cells that takes place during the delay in development. Driesch (’95) found that pieces of the blastula wall of the sea-urchin, _if large enough_, can also produce a gastrula and embryo. I found that the number of cells in these pieces does not increase appreciably after they are cut off (if the operation has been carried out at the end of the cleavage period), and that the new embryo is organized out of the cells present at the time of removal of the piece from the wall. There is, therefore, in this case no chance for “post-generation” by means of new cells produced at the side, which Roux has supposed to take place in the frog embryo. The development of pieces of the blastula wall, if they are not too small, also shows that the lack of power to develop, found in some of the one-fourth and in many of the one-eighth blastulæ, is not the result of any special differentiation that they have undergone during the cleavage period, but is due to their size. A recent series of experiments by Driesch (1900) on the development of isolated blastomeres of the sea-urchin’s egg has given more exact data in regard to their limit of power to produce embryos, and has shown the possibilities in these respects of different parts of the egg. By means of a method discovered by Herbst (1900) it is possible to obtain isolated blastomeres more readily than by the somewhat crude shaking process. If the eggs, after fertilization and after the removal of the membrane by shaking, are placed in an artificial sea water, from which all calcium salts have been left out, the eggs divide normally, but the blastomeres are not held firmly together, and readily fall apart if the egg is disturbed. By means of a fine pipette any desired blastomere or group of blastomeres can be picked out. If these are returned to sea water they continue to develop.