PRENATAL GROWTH OF THE PIG 127 



at 18 mm., where it forms 1.87 per cent of the total. Decreas- 

 ing, at first rapidly, then more slowly, at 156 mm. it reaches its 

 minimum prenatal relative sizej about 0.21 per cent of the total. 

 Throughout the remainder of the stages examined, it averages 

 about 0.25 per cent. In the late fetus, however, it forms 0.33 

 per cent, while in the adult it forms 0.043 per cent of the total. 



In the first stage observed, its weight is about one-fifth that of 

 the brain, later in the fetal period, about one-twelfth to one-fif- 

 teenth, in the late fetus reaching one-twelfth. In the adult the 

 weight of the cord is about one-half that of the brain. Its rela- 

 tive growth rate must be much greater in postnatal life than is 

 that of the brain; or, otherwise expressed, the decrease in rela- 

 tive size of the brain is much greater than that of the cord. 



The cord is relatively larger in the human fetus than in the pig 

 in the earlier stages, but smaller in the later stages. Its maxi- 

 mum in the human embryo is 4.85 per cent m the fifth week. 

 At 17 mm., it forms 3.43 per cent, in comparison with 1.87 per 

 cent at 18 mm. in the pig. At birth, the human cord forms about 

 0. 15 per cent of the total weight, which is only one-half the rela- 

 tive size of the pig's cord. In the human adult, the cord forms 

 0.06 per cent of the total (Vierordt), which is almost twice that 

 of the pig. The conditions regarding comparative prenatal and 

 postnatal growth of brain and cord are similar in both human and 

 pig; that is, the brain has the more rapid relative growth rate in 

 prenatal life. In postnatal life the relative growth rate of the 

 cord is more rapid, as Donaldson has pointed cut for the human. 



Figures from Donaldson ('08) show that at birth the spinal 

 cord in the white rat forms, on the average, . 73 per cent of the 

 total, while in the adult (10-12 months), it forms about 0.20 

 per cent. The decrease is therefore similar to that in the pig and 

 human. Data by Welcker indicate a similar decrease for the 

 chick, but not for the dog. 



The eyeballs. {Table 1: fig. 3). The eyeballs, starting with 

 an average relative size of 0.14 per cent of the total at 15 mm., 

 increase rapidly to a maximum of 1 . 15 per cent at 86 mm. From 

 this stage, they decrease slowly until, at birth, they form .41 per 



