FORM AND GROWTH 127 



26 per cent of the body weight. There is a similar postnatal in- 

 crease in the chick, although the relative weight of the skin 

 without the feathers remains nearly constant. 



The difference between the sum of the relative (percentage) 

 weights of all the organs weighed and the total body weight 

 (100 per cent) is indicated in the table by the last item, "Re- 

 mainder." This item includes various smaller unweighed struc- 

 tures, intestinal contents, fat and connective tissues, loss of fluids, 

 etc. This factor in the newborn appears fairly uniform in all 

 except the human. In the adults, however, it varies from 11.5 

 per cent in the dogfish to 32.3 per cent in the guinea pig. It must 

 be kept in mind that variations in this amount will necessarily 

 affect the relative weight of all other parts. This, for example, 

 will account for some peculiarities in the relative size of organs 

 in the guinea pig, as compared with the other species. These 

 differences are perhaps more clearly apparent in the graphs for 

 the various systems. 



Some graphs will now be presented to illustrate the changes 

 in the relative size or weight of various parts in typical verte- 

 brates during prenatal and postnatal life. In these graphs, the 

 time element is disregarded, and the purpose is merely to show 

 the variation for different species at certain stages: in early 

 embryonic life, at the time in the prenatal period when the 

 parts reach their maximum relative size, at birth, at the period 

 of postnatal maximum relative size, and in the adult stage. The 

 data in many cases are not strictly comparable, and are only ap- 

 proximate ; but they will serve to indicate roughly similarities 

 and differences in the relative growth of various parts of the 

 body. 



The relative weight (or volume) of the head is shown in 

 Figure 70. The human head increases from about 35 per cent 

 of the body in the earliest embryo measured to a maximum of 

 45 per cent somewhat later. Thereafter it grows more slowly 

 than the body as a whole, and decreases in relative size, forming 

 about 25 per cent at birth and only 7 or 8 per cent in the adult. 



