SECT. 2] AND WEIGHT 437 



on the average. Moreover, Janisch treats the same curve as a catenary 

 exponential one. And, ahhough the " Wachstumskonstante " for 

 the various organs and parts of the embryo show differences which 

 might well be regarded as characteristic for the tissue in question, it is 

 disturbing to find so wide a difference from the predicted value in 

 the case of the rat embryo, explained though it is by Schmalhausen 

 as due to variable factors in the food of the maternal organism. The 

 reason why 300 is the number to which these figures approach is, 

 of course, because, according to Schmalhausen's formula, the in- 

 crease of the embryonic weight can be expressed by the equation 



W= k [atf, 



where W is the weight, a the "Lineargrosse", t the time and k a 

 constant. This agrees with the hyperbolic nature of the Cvjt curve. 



Table 57. Instantaneous percentage growth-rate [Chick). 



for the equation of an equal-sided hyperbola is j; = 3/x. Schmal- 

 hausen does not derive his Cv directly, but calculates it in each case 

 from the Minot percentage growth-rate figures. It is instructive to 

 place side by side the instantaneous percentage growth-rates of Brody 

 and Schmalhausen for the chick embryo, as is done in Table 57. 

 That of the former has three constant periods, that of the latter 



