SECT. 2] AND WEIGHT 445 



embryo, during which one type of chemical molecule is predominantly 

 burned to furnish energy for the growing organism. This subject will 

 be handled fully later (Sections 6-8, 7-7, and 9-5) ; here it suffices to say 

 that the beginning of development is in many ways closely associated 

 with an important carbohydrate metabolism, and the latter part 

 with the metabolism of fatty acids, while the intermediate part would 

 appear to have an association with catabolism of protein, in view 

 of the fact that the point of maximum protein catabolism occurs 

 when 8*5 days of development have been completed. These periods, 

 in Schmalhausen's view, may be identified with those in which he 

 finds spurts in the growth-rate. A certain amount of scepticism 

 about this identification would seem justifiable until we have 

 irrefragable proof that the spurts are more than chance variations 

 in a curve composed of too few data. His suggestions involve 

 the view that "Abbauprodukte" accumulate from time to time 

 in the developing embryo, and so hinder its growth (essentially 

 the same theory as those of Jickeli and of Montgomery) . Thus 

 his first depression of the growth on the 4th day corresponds to an 

 accumulation of lactic acid and ammonia (see further on for the 

 detailed references) and his second depression of growth on the 

 gth day corresponds to an accumulation of urea. Finally, his 

 third depression of growth about the 12th day corresponds to an 

 accumulation of uric acid. He admits that there is nothing chemical 

 which obviously coincides with the later depressions of growth, but 

 supposes that they depend on the decreasing excretory power of the 

 mesonephros. After the i6th day the metanephros would be under- 

 taking the duty of excreting waste products, and growth accordingly 

 begins again. Ingenious as these correlations are, they cannot be 

 said to be convincing, in view of the fact that many other processes 

 besides the excretion of waste products may be supposed to be exercis- 

 ing an eflfect on the growth-rate. More interesting is Schmalhausen's 

 attribution of great importance to the surface of the blastoderm, 

 the blastodermal capillaries, and the active surface of the excretory 

 organs. Measurement of these during the course of development 

 would throw a bright light on these problems. Schmalhausen did 

 himself compare the growth in weight of the embryonic kidneys 

 with the daily increment of the whole body, and, although the figures 

 were rather erratic, he felt able to conclude that, owing to the slow 

 growth of the mesonephros and metanephros, the excretory surface 



