Driesch (’95) reëxamined the development of isolated blastomeres in one of the ascidians, _Phallusia mammalata_, and found that the cleavage of blastomeres, isolated by shaking, is neither like that of the whole egg, nor is it like that of half the normal cleavage, although it shows some characteristics of the latter. A symmetrical gastrula is produced, and from this a typical whole larva of half size. These larvæ lack, however, one or more papillæ, and the otolith rarely develops. The absence of these organs Driesch ascribes to the rough treatment that the egg has received, since embryos from whole eggs may sometimes lack these organs if the development has taken place under unfavorable conditions. The isolated one-fourth blastomere may also produce a whole larva. Crampton (’97) has also studied the development of the isolated blastomeres of another ascidian, _Molgula manhattensis_. He has more fully worked out the cleavage, and finds that the isolated blastomere segments as a part, _i.e._ as it would have segmented had it remained in connection with the rest of the egg. In general appearance the half-cleavage seems to differ from the half of the complete cleavage, because rearrangements of the blastomeres occur, but despite these shiftings the form of the division is always like that of a part. A whole embryo develops, although there may be defects in certain organs, which are due, he suggests, to the smaller amount of material available for the development of the larva. Zoja showed in 1894-1895 in a number of jellyfish that the isolated blastomeres produce whole larvæ of smaller size.[121] In one form, liriope, the endoderm that forms the digestive tract is normally delaminated at the sixteen-cell stage, each cell of the blastula wall dividing into an inner and an outer part. In the blastula from the one-half blastomere this delamination also takes place when sixteen cells are present, and not at the preceding cleavage when only eight cells are present. In this form, therefore, the whole number of cells develops before the delamination takes place, and the one-half larva is composed of the same number of cells as is the normal embryo at this stage, but the cells are only half as large. In other species the endoderm appears to begin to develop in the half-larvæ when only half the total number of cells is present.