244 Chas. W. and G. T. Hargitt. 



have assumed their definite shape and relationship and the 

 nettling cells have formed from the more deeply lying ectoderm 

 cells. The entoderm, continuing its differentiation, has come 

 to lie closely against the ectoderm again. The cells do not be- 

 come so long and narrow as the ectoderm and the cytoplasm is 

 more vacuolated than in the ectoderm cells. The yolk is almost 

 used up; the last portion remaining is limited to the entoderm 

 cells of the anterior end of the planula. At the posterior end 

 (lower in the figure) the ectoderm is already differentiating into 

 what will become the point of attachment of the planula. The 

 entire surface is covered with cilia, not represented in the figure. 



The formation of the definitive ectoderm and entoderm is, 

 then, only a differentiation in size, shape, etc., the position of the 

 individual cells remaining relatively the same. There is never 

 a solid mass of cells in the embryo such as one finds in the early 

 planula of the Hydromedusae, and the coelenteron is present from 

 the first as the remains of the old archenteron of the gastrula. 



Reference to the historical section will show that the difference 

 in the results of this study and those of other investigators upon 

 Scyphomedusse are chiefly differences in detail. The irregularity 

 and inequality of cleavage shown by Glaus (1883),Haeckel (1881), 

 Metschnikoff (1886), Hyde (1894) and others to be variable is 

 here confirmed and shown to be even more variable. In some 

 Hydromedusae irregularity is the rule. However in these Scy- 

 phomedusse the end result is the same whether irregularity or 

 regularity is the chief character of the cleavage, and the varia- 

 tions and differences are then of only very minor importance. It 

 has also been pointed out by many workers that considerable 

 variation occurs in the process of gastrulation, but here again the 

 end result is the same, viz., a typical planula, so that the dif- 

 ferences are evidently of little significance. The methods of 

 attaining the same result, though different, may only be indica- 

 tions of the individual differences of the eggs or of the environ- 

 ment as Gonklin (1908) pointed out. The controversies over these 

 unessential points only tend to show that attempts to bring these 

 and other processes into exact and constant agreement with some 

 assumed ' law ' are futile, and often only indications of ignorance 

 or bias. 



