58 



ohim;eroid fishes and their development. 



(fig. 60), for it shows that while the cleavage lines have failed to express themselves 

 on the dorsal side of the egg, they yet appear conspicuously on the vegetal side. Thus 

 the line in fig. 59 at a passes sharply inward (fig. 60), subdividing into a series of 



r7 , 58 59 



Figs. 57-61. — Elggs showing progressive cleavage of the yolk mass. In a blastula (57) a conspicuous fissure 



is noted between the points * and *. 



cleavage lines which in turn merge with other cleavage lines {b-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 deepl}- 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 yolk. It is evident, accordingh', that in this stage the egg 

 is being divided up 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 the egg — caused them to 

 round out their contours to the degree shown in fig. 61. 



