. Carnegie Institution of Washington publication. . Figs. 57-61.—Eggs showing progressive cleavage of the yolk mass. In a blastula (57) a conspicuous fissure is noted betv^een the points * and *. cleavage lines which in turn merge with other cleavage lines {h-g) passing downward and inward from the equatorial zone of the egg. In some cases well-marked yolk masses are outlined, as at the point marked with an asterisk (*), suggesting large yolk-filled blasto- meres on the ventral wall of an amphibian blastula. It may be remarked that the lines here described are not mere surface mark- ings, for
. Carnegie Institution of Washington publication. . Figs. 57-61.—Eggs showing progressive cleavage of the yolk mass. In a blastula (57) a conspicuous fissure is noted betv^een the points * and *. cleavage lines which in turn merge with other cleavage lines {h-g) passing downward and inward from the equatorial zone of the egg. In some cases well-marked yolk masses are outlined, as at the point marked with an asterisk (*), suggesting large yolk-filled blasto- meres on the ventral wall of an amphibian blastula. It may be remarked that the lines here described are not mere surface mark- ings, for during the process of hardening an egg, c. g., in acetic- sublimate, one may separate the yolk masses by aid of dissecting needles, and in this process it becomes clear that the lines are in reality fissures dipping deeply into the substance of the egg. Indeed, in the former specimen it was found that the mass marked with an asterisk (*) could be removed en bloc from the remaining mass of 3-olk. It is evident, accordingly, that in this stage the egg is being divided uji on its ventral side into a number of large yolk masses; that these masses stand in relation to the entire egg very much as do, e. g., in the frog's egg, the blastomeres of the lower pole to this entire holoblastic egg; further, that the fissures which accomplish this result, like cleavage lines on the vegetal side of the holoblastic egg, are interconnected with a series (a-g, in fig. 60) of cleavage lines which pass downward and inward from different points in the equatorial region of the egg. Between the stages shown in figs. 59 and 60, and those in figs. 61 and 27, which are older by about nine days, observations are lacking. It is nevertheless clear, by comparison of these stages, that the yolk masses shown in fig. 60 have separated from one another widely as the fissures deepened, and that, as the masses became more distinct, their condition of surface tension—in view always of the syrupy consistency of
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