Driesch (’95) has also made a study of the potentialities of the blastula and gastrula stages of sphærechinus, echinus, and asterias. If a blastula is cut in half before the mesenchyme cells are produced, both pieces produce gastrulæ and larvæ. Since some of the pieces probably come from the animal hemisphere, and others from the vegetative hemisphere, it follows that all parts of the blastula possess the power of producing whole embryos, and in this respect the potentialities are the same as for the blastomeres. If the experiment is made at a stage just before the archenteron has begun to develop (Fig. 65, _A_), the results may be different. A half that contains the region from which the archenteron is about to develop will produce a gastrula and a larva (Fig. 65, _A_, lower row to right of _A_). A half that contains only the opposite regions of the egg (Fig. 65, _A_, upper row) may in some cases gastrulate,[119] often abnormally, but as many as half of the pieces do not gastrulate. They may remain alive for a week or more, and even produce a typical ciliated ring with a mouth in the centre, but do not form a new archenteron. These important results show that after the formation of the mesenchyme and archenteron at one pole, the other cells of the blastula wall are no longer able to carry out a process that the same cells were able to carry out at a slightly younger stage, but whether this loss of power is connected with the previous formation of the archenteron, or due to some other change which has by this time taken place in the cells, cannot be determined from the experiment. It is also important to note that these small ectodermal blastulæ can still develop whole, typical, ectodermal organs, the ciliated ring and the mouth, and that the former especially has the characteristic structure of the whole normal ring. [Illustration: FIG. 65.--_A._ Blastula of sea-urchin beginning to gastrulate. Cut in half as indicated by line. Two rows of figures to right show development of upper and lower halves. _B._ Later gastrula cut in half. Two rows of figures to right show later development. _C._ End of gastrulation process. Embryo cut in half. Two rows of figures to right show later stages of each half. _D._ Formation of endodermal pouches from inner end of archenteron. Embryo cut in two. Two rows of figures to right show later stages.]