128 



GROWTH 



The head of the pig shows a smaller prenatal increase, and re- 

 mains relatively smaller than the human head at all stages of 

 the life cycle. The dogfish head is relatively slightly smaller 

 than the pig's head in the early embryo, but increases to a 

 much larger maximum relative size soon afterward. The dogfish 

 head likewise remains relatively smaller than the human head 

 throughout the prenatal period. (This dogfish, Mustelus cants, 



is a viviparous form.) The 

 head of this fish reaches its 

 adult proportion, about 12 per 

 cent of the body weight, at a 

 comparatively early postnatal 

 stage, however. This is rela- 

 tively larger than the adult 

 human head. Quantitative 

 data for the guinea pig, chick, 

 and rat are available for the 

 postnatal period only. The 

 relative size of the head in the 

 guinea pig appears closely 

 parallel with that in the dog- 

 fish. In reality it does not 

 reach the minimum until adult 

 size in the guinea pig, while 

 the adult proportion is reached 

 much earlier in the dogfish, as 

 before mentioned. The head in the postnatal chick is relatively 

 much smaller than in the other species. It is also characterized 

 in the adult by a distinct sex difference. The head is relatively 

 larger in the cockerel, due chiefly to the comb and wattles. The 

 rat shows a marked peculiarity which has not been noted in the 

 other species. The maximum postnatal relative size of the head 

 occurs, not at birth, but at a slightly later period, after which it 

 decreases gradually to the typical adult relation. 



Some data for the relative weight of the moist ligamentous 



Head - Pe/dtiw Weight 



Figure 70. Graph showing changes in the 

 relative weight of the head in different 

 species at various stages, prenatal and post- 

 natal. Data from Bessesen and Carlson, 

 Kearney, 27 Jackson, 20 ' 22 Jackson and 

 Lowrey, 20 Lowrey, 35 and Latimer. 31 



