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.
The conditions in the egg of the bony fishes are very different from those in the preceding forms. The protoplasm, from which the embryo is produced, accumulates at one pole to make the blastodisc. After the cleavage of this blastodisc, the blastoderm that has resulted grows over the yolk sphere at the same time that the embryo is forming along one meridian. I carried out some experiments, in 1895, on the eggs of Fundulus heteroclitus. If one of the first two blastomeres of the egg of fundulus is destroyed, the remaining one produces a whole embryo. If three of the first four blastomeres are removed, the remaining one may produce a whole embryo of small size. The problem of development is, in the case of the fish, different from the other cases described, inasmuch as the whole yolk sphere is left attached to the remaining blastomere and is covered over by cells derived from this blastomere. The smaller embryo that is formed lies on a yolk of full size.[122]
Wilson’s work on amphioxus has been already described in connection with the experiments on the sea-urchin’s eggs. Later I (’96) also obtained whole larvæ from one-half and one-fourth blastomeres, and I also found that the one-eighth blastomeres do not develop beyond the blastula stage. The number of cells of which the one-half larva is composed is half that of the normal larva, and the one-fourth larva is made up of one-fourth of the total number of cells.
Fig. 66.—Ctenophore-egg and embryo. A. Normal sixteen-cell stage. B. Half-sixteen-cell stage. C. Later half-segmentation stage. D. Later half-embryo. E. Corresponding whole embryo. F. Half-embryo seen from side. G. Same seen from apical end. In F and G, four rows of paddles present, three endodermal sacs and ectodermal stomach.
In all the preceding cases in which the blastomeres have been separated, a whole embryo has developed, although the cleavage was often like that of a part. In one form, however, it has been found that a whole embryo does not develop. Chun (’92) first showed that the isolated one-half blastomere of the ctenophore egg produced a half-larva. He also inferred from certain incomplete embryos caught in the sea, that these incomplete larvæ could subsequently regenerate the missing parts. Driesch and Morgan (’95) studied the development of the isolated blastomeres of another ctenophore, Beroë ovata. They found that the isolated one-half blastomere divides exactly as a half of the whole egg ([Fig. 66], A, B, C). It remains more or less a half-structure, even after the ectoderm has grown over the whole surface ([Fig. 66], D). The invagination of ectoderm, to form the so-called stomach, that takes place at the lower pole of the whole embryo, is formed at one side of the lower pole in the half-embryo ([Fig. 66], F, G). It pushes into the endodermal yolk mass, and lies not in the middle, but somewhat to one side. In the normal embryo there are formed four endodermal sacs or pouches in the central yolk mass that become connected with the inner end of the ectodermal stomach, around which they lie symmetrically. In the half-embryo two sacs are formed, and in addition a smaller third sac, which always lies on the side of the stomach that is nearest the outer wall ([Fig. 66], F, G). The embryo is, therefore, somewhat more than half the normal embryo in regard to the number of its endodermal sacs.
There are present eight meridional rows of paddles in the normal embryos of the ctenophore. They lie symmetrically on the sides, converging towards an apical sense organ. In the one-half larva there are always only four of these rows of paddles that are not equally distributed over the surface, since on one side there is a wider gap between two of the rows than elsewhere ([Fig. 66], G). The sense plate also lies somewhat eccentrically, i.e. more towards the side corresponding to that at which the other blastomere lay.
If the one-fourth blastomeres are separated, each continues to segment as though still a part of the whole. A one-fourth embryo develops that has an unsymmetrical stomach, with two endodermal sacs. There are only two rows of paddles. The embryos are, therefore, in several respects one-fourth embryos, but the presence of two endodermal sacs, instead of only one, shows that in this particular, at least, the embryo is more than a fourth of the whole.