SECT. 2] AND WEIGHT 441 



inevitable in those days — to recognise the essential difference be- 

 tween these movements of masses and the molecular processes which 

 precede and accompany them and which are characteristic of another 

 order of magnitude." 



The papers of Schmalhausen are of much importance in this 

 matter. Inspired by the views of His, who declared in 1874 that all 

 the development of shape could be ascribed to unequal growth in 

 various component parts of the embryo, he set himself to weigh and 

 measure a great number of these individual sections. 



He first studied the relative growth-rates of the brain and 

 eye of the chick embryo, together with the liver, lung and stomach, 

 representing the organs of endodermal origin. In each case, he 

 calculated the % growth-rates and the percentages formed of the 

 weight of the whole body. For the organs of mesodermal origin, 

 he chose the heart, the mesonephros, the metanephros, the ovary, 

 and testis. These figures he treated in the same way. In many cases 

 his weights were not obtained directly but by reconstructing from 

 serial sections and then weighing, proper allowance being made for 

 complicating factors such as specific gravity. Fig. 56 shows one of 

 his graphs — it is specially interesting as showing the definite decrease 

 in weight which the mesonephros undergoes after the 15th day in 

 giving place to the metanephros or adult kidney. It also includes 

 % growth-rate curves for the fore and hind limbs. Lastly, he 

 ascertained the growth-rate of the feathers. 



In general, he found that the changes in the growth-rates of organs 

 were synchronous. The percentage growth-rate (see Fig, 57) seemed 

 to have peaks in its descent, each one less marked than the pre- 

 ceding one. In each case, the growth-rate of every organ shows a 

 certain rise, but the amount of the rise differs in different cases — thus 

 the lung is the organ which is growing fastest about the 6th day, the 

 hind extremity about the loth day and the stomach about the 13th. 

 On the whole, the periods of depression of the growth-rate of the 

 majority of organs are from 7 to 9 days, from io| to ii| days, and 

 from 14 to 16 days. When the weights of individual organs, however, 

 were arranged plotted against weight of embryo, not age, the peaks 

 disappeared, as would be expected, for the total weight is the sum of 

 the weights of the organs. It would be interesting to plot the logs, of 

 Schmalhausen's organ-weights against age in order to obtain the 

 instantaneous growth-constants of Brody for each one. Schmal- 



