74 HOMOIOTHERMISM 



raw food materials, and (4) the rate of nuclear synthesis. 

 He characterizes the process which takes place at the 

 slowest rate — and therefore limits all others — as *Hhe 

 master reaction." From his point of view, growth may- 

 be slowed up by the exhaustion of antocatylist or food, 

 or by the accumulation of products. 



Brody (1927) has made extensive studies of growth. 

 ** Growth curves consist, in all cases, of two major seg- 

 ments. The first major segment is, in the case of higher 

 animals and plants, made up in turn of several (probably 

 five) shorter segments during each of which growth takes 

 place at a constant percentage rate. The transitions be- 

 tween the successive stages are abrupt, the abruptness 

 being of the same order as that of metamorphosis in cold- 

 blooded animals. . . . The junction between the two 

 major segments occurs at puberty in animals and flower- 

 ing in plants. . . . The two major segments of the in- 

 flection are not symmetrical about the major inflection. 

 The slope of the segment following the inflection is al- 

 ways less than the slope of the segment preceding the 

 inflection. The major inflection does not occur in the 

 center of the growth curve.'' At the beginning, growth 

 may be 100 to 200 per cent per day. The body weight 

 may be doubled in from 7 to 17 hours. Two months after 

 conception the increase in man is about 8 per cent per 

 day. **Dr. Stockard called attention to the fact that the 

 peak in the mortality curve of the chick at 5 days is the 

 counterpart of the peak in the prenatal mortality curve 

 in man at 3 months. This is the junction between the 

 embryonic period (formation of organs) and the fetal 

 period (enlargement of body and organs)." The nature 

 of the growth in the two periods is quite different, hence 

 it is not strange to find a high mortality at the time of 

 transition from one to the other. 



