The Gastrulation of the Egg of Bufo lentiginosus
AN examination of the literature dealing with the early development of the amphibian egg shows many conflicting observations and theories regarding the origin of the blastopore and the manner of formation of the mesoderm and notochord. It is evident, therefore, that more forms must be studied and the observations in some cases carefully repeated before any general conclusions regarding the origin of these structures can be drawn for the entire group. The amphibian eggs that have been most
... ave been most studied are those of Rana, Triton, Axolotl, and Bombinator. The present paper deals with the early development of the egg of Bufo lcnt1zr<inlosuls from the end of cleavage until the closure of the blastopore. The eggs of this amphibian, although very abundant and easily obtained, have been but little used either for observation or for experiment, owing, doubtless, to their small size and to their relatively deep pigmentation, which makes it very difficult to follow the fate of living cells. iVIETHOD. In preparation for sectioning, eggs were killed in various fluids: picro-acetic, picro-sulphuric, formalin, and corrosiveacetic. By far the best results were obtained with corrosiveacetic (5 per cent acetic). Formalin (3-10 per cent) gives exceedingly good preparations for a study of surface structures, but it cannot be relied on to give histological details, as it usually produces numerous cracks in various parts of the egg. The sections were stained on the slide with the mixture of borax carmine and Lyon's blue, recommended in a previous paper (King, io). With the use of this stain the 527 This content downloaded from 184.171.THE 4IZ7ERICAN NA TURALIST. [VOL. XXXVI. nuclei appear red, the yolk blue, and the cell outlines are brought out with great clearness. THE FORMATION OF THE BLASTOPORE. The egg of Bijo lctwZig-inosus probably contains a greater amount of pigment in proportion to its size than that of any other common amphibian. It is, therefore, very difficult to study the movements of individual cells before and during the formation of the blastopore, as has been done to some extent in the eggs of several other species of amphibians, where the pigmentation is less extensive and cell outlines can be readily determined. In the egg of Bufo the pigmentation extends in all cases some distance below the equator, and I have frequently found eggs in which fully three-fourths of the surface was deeply pigmented before the appearance of the blastopore. Individual eggs, even from the same female, differ greatly in the amount of pigment they contain. As a rule, the pigment line extends farther down on one side of the eg-g than on the other, as seen in Fig. I , agreeing in this respect with the frog's egg according to Schultze (24), Morgan and Tsuda (r7), and Wilson (27). Sections through an egg at the close of the blastula stage show a large segmentation cavity in the upper hemisphere. Its dorsal wall is formed, as in the frog, of three or four layers of small angular cells of uniform size. The cells forming the outer surface of the egg are almost completely filled with pigment granules, and a considerable amount of pigment is scattered throughout all the cells of the upper part of the egg. The yolk cells below the segmentation cavity are much larger, more rounded, and stain less intensely than the cells in the upper hemisphere. There is the same gradual increase in the size of the cells from the upper to the lower pole that other investigators have noted in the frog's egg. The dorsal lip of the blastopore invariably appears some distance below the equator of the egg, but never in the middle of the lower hemisphere, as maintained by Houssay (8) for the axolotl and Jordan (9) for the newt. This content downloaded from 184.171.