130 



GROWTH 



Prenatal 

 20 



Natal 



•S 15 





Maximum 

 postnatal 



Adult 



\ 

 \ 

 \ 



\ 



N 



period, the relative weight of the musculature in the dogfish is 

 less than at birth. This is apparently true also for the other 

 forms, although quantitative data are scarce. 



The progressive increase in the relative size of the muscula- 

 ture, together with its large bulk, accounts in part for the tend- 

 ency to decrease in the relative size of other structures, such as 



the nervous system and the 

 viscera. The marked decrease 

 in the relative size of the brain 

 (which parallels that of the 

 head) is shown in Figure 73. 

 At the beginning, the chick 

 brain seems to be relatively 

 the largest of all, though exact 

 data for the earlier embryos 

 are lacking. At the ninth day 

 of incubation the brain of the 

 chick forms about 28 per cent 

 of the body (Welcker and 

 Brandt), decreasing to less 

 than 3 per cent at hatching, 

 and to one-sixth of 1 per cent 

 in the adult. Aside from the 

 early chick, the human brain 

 appears relatively largest at 

 all stages, decreasing from 

 about 22 per cent in the embryo to 12 per cent at birth and 2.2 

 per cent in the adult. The brain of the dogfish is in the embryo 

 relatively larger than that of the pig; at birth the pig brain is 

 relatively larger, but in the adult they are nearly equal (less 

 than 0.1 per cent). The rat shows a temporary postnatal in- 

 crease in the relative size of the brain similar to that for the 

 head and skeleton. The guinea-pig brain (not shown in the fig- 

 ure) decreases steadily from 2.6 per cent of the body at birth to 

 O.044 per cent in the adult. 



Human 



Dogfish 



Rat 



— — Guinea pi<? 



Chick 6" 



Chick $ 



5keleton 



EeldtiueWtifht 



Figure 72. Graph showing changes in the 

 relative weight of the skeleton in different 

 species at various stages, prenatal and post- 

 natal. Data from Bessesen and Carlson, 

 Jackson, 20 ' 22 Jackson and Lowrey, 20 Kear- 

 ney, 21 and Latimer. 31 



