Regeneration

by Thomas Hunt Morgan

The smallest blastomeres that produce gastrula are the one-sixteenth products. Out of a total of 139 cases only 31 produced true gastrulæ, 5 produced gastrulæ with evaginated archenteron, and 103 remained blastulæ with long cilia. The one-thirty-second blastomeres were not observed to gastrulate.

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.

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.

Similar phenomena have been made out by Driesch in the development of the archenteron of the same forms. At the end of the normal gastrula period of the starfish embryo, there is produced from the inner part of the archenteron two outgrowths, or pouches, that later constrict off to give rise to the cœlom sac and water-vascular system. If the gastrula is cut in two in such a way that the inner end of the archenteron, i.e. the part from which the pouches develop, is cut off ([Fig. 65], C), it is found that the piece containing the posterior part of the archenteron closes in, forms a new sphere, and from the present inner end of the archenteron (that has also healed over) a pair of pouches is produced ([Fig. 65], C, lower row to right of C). These pouches have arisen, therefore, from a more posterior part of the archenteron than that from which the pouches normally arise.

If the same experiment is made at a later stage, when the pouches have been given off from the archenteron ([Fig. 65], D, lower row to right of D), no new pouches are formed. This means that after the archenteron has once produced its pouches it loses throughout all its parts the power to repeat the process, although these parts possessed this power at an earlier stage. It is a very plausible view that the result is directly connected with the formation of the normal pouches, although it is of course possible that some other change has taken place in the archenteron that prevents the formation of the pouches.

In order to give as nearly as possible a consecutive account of the experiments on the eggs of the frog and of the sea-urchin, a number of other discoveries have been passed over. Let us now examine some of the results on other forms.

Chabry, as early as 1887, experimented with the eggs of an ascidian. By means of an ingenious instrument he was able to prick and kill individual blastomeres. The results of his experiments were not described very clearly, and later writers have interpreted his results in different ways.[120] Chabry stated that he obtained half-embryos from one of the first two blastomeres, but his figures show, especially in the light of later work, that the embryos were whole embryos of half size, although certain organs, as the papillæ and the otolith, may be lacking.

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.