SECT. 2] AND WEIGHT 449 



and his associates. A rich harvest awaits the investigator who dis- 

 covers the relation between chemical constitution and the differential 

 growth-ratios. Perhaps a fruitful line of work will develop from the 

 finding of Robb that the log. weight of an organ plotted against the 

 log. body-weight often gives a straight line. He has suggested that 

 organ-growth may depend on a kind of partition-coefficient, organs 

 competing, as it were, for the building-stones in the blood-stream, 

 and securing now a greater now a lesser proportion, according to 

 the changing permeability of their cell-walls. 



The changes which occur in the chick's relative growth-rates of parts 

 at hatching have been studied by Latimer, who combined together 

 the data collected for pre-natal stages by Schmalhausen and those for 

 post-natal stages by various American workers. His results lead to 

 the conclusion that the organs and parts fall into three groups : 



(i) Those in which no change in relative growth-rate is found, 

 e.g. liver, gizzard, feathers, ovaries. 



(ii) Those which show a brief post-natal retardation, e.g. total 

 body-weight, brain and heart. 



(iii) Those which show a marked post-natal acceleration, e.g. 

 kidneys and spleen. 



As will be shown below, the brain and eyes, so prominent in the 

 embryo, fall consistently throughout life when expressed as per cent, 

 of the whole weight, while the gizzard, liver, kidneys, spleen and 

 heart have a maximum in early post-natal life. 



Other work on relative sizes of parts has been done by Jenkinson 

 on embryonic trout, by Keene & Hewer on man, and by Jackson 

 who gives a graph (Fig. 59) showing the relative proportions in the 

 human embryo at different stages of its development, collected from 

 all the available data. Boyd (on man) and Welcker & Brandt (on 

 the chick, salamander and man) made earlier attempts at the same 

 thing, but the ages of their embryos were unknown. The graph 

 demonstrates the relatively large size of the brain in the early sizes, 

 and in many ways resembles the graph for the organs of the chick 

 given by Schmalhausen. "In general," says Jackson, "the period 

 of maximum relative growth passes in a somewhat wave-like manner 

 over the body from the head towards the foot. The head reaches 

 its maximum relative size about the 2nd month. In the trunk, the 

 upper portion, including the thorax and the upper abdominal 

 viscera, is relatively largest throughout the earlier half of foetal life. 

 NEi 29 



