438 ON INCREASE IN SIZE [pt. m 



shows a gradual decline, and the figures illustrate what has already 

 been said, namely, that, until we possess much better statistical data 

 than is actually the case, we cannot differentiate between the 

 Brody position and the Murray-McDowell-Schmalhausen position.* 

 As the matter is fundamental in view of the important theoretical 

 issues involved, the accumulation of more data is urgently to be 

 desired. It may be mentioned that Cohn & Murray, plotting log, 

 weight/age curves for the growth of embryonic heart cells in tissue 

 culture, obtained curves concave to the age axis and not straight lines. 

 Schmalhausen also studied the growth in length of the chick 



embryo, calculating it from the weight by the formula L = VW. 

 The daily gain in length a he found to be variable around a constant 

 value of 1-47 for the first half of development and 2-00 for the second 

 half. But when the weights were corrected by the estimation of the 

 embryo's specific gravity (average for first half 1-025, average for 

 second half i -06) the corresponding daily gains in length worked out 

 at I -Go and 1-79. A further correction made necessary by the presence 

 of the feathers during the last half of development brought the figure 

 down to 1-64, so that throughout incubation the embryo apparently 

 grows in length at the same average rate. The duck embryo, ac- 

 cording to Schmalhausen, has a daily length increment of i*io mm., 

 and this value he regards as constant for the species. He went on 

 to calculate a for the human embryo, using the weight data of 

 Friedenthal and Zangemeister, and for the embryo of the white rat, 

 using the data of Stotsenberg. All these results are shown in Table 58, 

 together with his further assessments of a calculated from the " Nor- 

 mal tafeln" of Minot & Taylor for the rabbit and Keibel for the pig. 

 The daily size ("Lineargrosse") increments of his own measurements 

 of separate organs and parts of the chick embryo are also given. 

 During the course of development the value of a rises and falls 

 according to the rate of growth. If the value a/ 1 is calculated, where 

 a is the daily increment in length and / the length of the part in 

 question at the beginning of the period, the absolute size of the 

 part will cease to affect the result, and the organs will be comparable 

 with themselves and with the whole embryo. When this is done, the 

 ratio is found to be fairly constant, rising as high as 10-3 per cent, 

 for the stomach and falling as low as 6-3 per cent, for the lens. These 

 figures are also given in Table 58. Schmalhausen concluded from 



* Recent work by Byerly supports that of Brody. 



