FORM AND GROWTH 



129 



"Po3rnaral 



Adult 



skeleton are shown in Figure 7 1 . Weights for the prenatal pe- 

 riod are available only for the dogfish. In this case, there ap- 

 pears to be a slight increase in the relative weight of the skele- 

 ton, reaching a maximum of 10 per cent shortly after birth, and 

 decreasing slightly in the adult. The guinea pig presents a some- 

 what greater relative decrease 

 in skeletal weight between 

 birth and adult stage. In the 

 other species, the skeleton at 

 birth appears relatively much 

 heavier. In both rat and chick, 

 there is a slight relative in- 

 crease after birth, with a 

 marked decline in the adult. 

 The chick shows also a distinct 

 sex difference, the skeleton 

 being relatively heavier in the 

 adult male. The human skele- 

 ton apparently remains nearly 

 constant in relative weight 

 during the postnatal period, 

 although but few observations 

 are available. 



Strikingly different rela- 

 tions appear in the muscula- 

 ture (Figure 72). One might 

 have expected a closer correla- 

 tion with the skeleton, on ac- 

 count of their functional relationship. The guinea pig appears 

 exceptional, with but slight change in the percentage weight of 

 the musculature during the postnatal period. The others pre- 

 sent a very striking increase. The musculature appears rela- 

 tively most poorly developed in the chick, intermediate in rat 

 and man (which have nearly the same percentage of muscle), 

 and relatively much greater in the dogfish. During the prenatal 



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Figure 71. Graph showing changes in the 

 relative weight of the musculature in differ- 

 ent species at various stages, prenatal and 

 postnatal. Data from Bessesen and Carlson, 

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

 ney, 27 and Latimer. 31 



