FORM AND GROWTH 135 



V. Relative Growth of Organ Parts and Tissues 

 It is evident from the foregoing data that the growth of the 

 body as a whole is the resultant of quite variable simultaneous 

 growth rates in various parts, systems, and organs. The law of 

 developmental direction evidently does not apply equally to 

 all the structures in a given region. A further analysis of the 

 growth process reveals a similar lack of uniformity even within 

 the individual organs. A few examples will be cited. In the hu- 

 man brain, it has been shown by Dunn 12 and Scammon 45 that 

 the fetal brain stem is relatively precocious in growth j while 

 the cerebellum at first lags behind, increasing in relative size 

 through late fetal and postnatal periods. Stewart 47 found a 

 similar relation in the postnatal growth of the brain in the rat. 

 Miller 38 observed marked differences in growth of the gray and 

 the white matter of the human spinal cord; and Scammon and 

 Armstrong 44 similarly noted varying growth rates for the con- 

 stituent parts of the eyeball. 



In the hypophysis (pituitary gland) of the rat, Jackson 24 

 found that during postnatal growth the pars anterior becomes 

 relatively larger, the pars posterior smaller, while the pars inter- 

 media remains unchanged in relative size. (Rasmussen 41 ' 42 and 

 Covell 9 found somewhat different relations in the human hy- 

 pophysis.) Similarly in the suprarenal gland, the cortex becomes 

 relatively much larger, the medulla correspondingly smaller 

 (Jackson 25 ). In the case of these two organs, the analysis was 

 carried still farther, showing different growth rates in the con- 

 stituent tissues and cells (Table II). In some regions there is 

 an increase in the relative amount of epithelial parenchyma, in 

 others a decrease, in comparison with the supporting tissue 

 (connective tissue stroma) and blood vessels. Finally the same 

 table includes some volumetric data on the corresponding cells, 

 showing changes in relative proportions of the cell nucleus and 

 cell body (cytoplasm). There is in all cases a marked reduction 

 in the nucleus-plasma ratio between newborn and adult stages. 

 Similarly variations in tissue and cell growth probably occur to 



