444 ON INCREASE IN SIZE [pt. iii 



of fore and hind limbs does not accomplish itself in the same spurts 

 as the viscera do, for during the 8th day depression in the growth- 

 rates of the latter the skeleton is growing vigorously, and during the 

 loth day peak it rather falls off. Very similar remarks apply to the 

 hind limb growth-rate. Schmalhausen concluded that very young 

 organs can respond to a given intra-embryonic environment by in- 

 crease in growth-intensity, while more differentiated organs can at 

 the same time respond by depressions in their growth-rates. "If one 

 and the same influence", he says, "can act in a stimulatory manner 

 on the growth of some parts or organs, and inhibitorily on the growth 

 of others, we can see how unequal growth can take place and hence 

 a change in form." It would also appear that the more development 

 goes on, the more different the rates of growth of different organs are. 

 Three factors seem to control the growth-rate of a single organ: 

 (i) the age of the embryo, (2) its own degree of differentiation, and 

 (3) growth-promoting substances or embryonic hormones present in 

 the circulation. Under (2) would be included the time of origin of 

 its "anlage" and the intensity with which its preliminary growth 

 would take place. These views are not compatible with Mehnert's 

 "laws of organogenesis", the main one of which was that the growth- 

 rate of an organ in the embryo was proportional to its degree of 

 development at the time in question. The only criticism that can be 

 levelled against Schmalhausen's work is that the number of embryos 

 employed was perhaps rather few. 



In conjunction with Stepanova, Schmalhausen made further in- 

 vestigations on the growth of the embryonic skeleton in the chick. 

 Similar fluctuations in pre-natal growth-rates of parts have been 

 discussed as regards the primates by Schultz. 



Schmalhausen has attempted to give an explanation of these 

 spurts in terms of metabolism. Summarised again there are, in the 

 case of the chick, three or four periods, in each of which the growth- 

 rate first rises and then falls, as follows: 



Days 



0—4 1st period, great fall from a high value 



4-9 2nd period, rising to the 6th day then falling 



9-12 3rd period, rising to the loth day then falling 



12-15 4th period, rising to the 13th day then falling 



15—21 5th period, rising to the 17th day then falling 



He has suggested that these periods may partly correspond to the 

 periods which can be distinguished in the development of the chick 



