126 LAWSON G. LOWREY 



Brain. {Table 1: fig. 1). The maximum relative size of the 

 brain (table 1) was found in the earliest stages examined (15 mm. 

 and 18 mm., accompanying the maximum relative sizes of the 

 head). It then forms, on the average, nearly 9 per cent of the 

 total body weight. The relative size then decreases, rapidly 

 in the earlier stages, then more slowly. In the fetus nearly full 

 term, it forms nearly 4 per cent of the total, and in the adult, 

 0.087 per cent. 



A comparison of the relative size of the head and brain shows 

 that, in the first stage examined, the brain forms about one-third 

 of the head, at 86 mm. about one-fifth, and throughout the later 

 part of fetal life and at birth, about one-sixth. In the adult it 

 forms only about one-seventieth of the head. 



This is a very small relative size when compared with the 

 human, both in prenatal and postnatal stages. The maximum 

 relative size for the human brain occurs during the second month 

 (at the same time as for the head), when the brain forms about 

 20 per cent of the entire body. This is about one half of the en- 

 tire head. At birth, the brain forms about 12 . 8 per cent or 14 . 6 

 per cent (still-born or live-born), which is about one-half of the 

 entire head. Vierordt estimates the adult human brain at 2.16 

 per cent of the total body, forming about one-third to one-fourth 

 of the head. 



According to figures from Donaldson ('08), the brain in the new- 

 born white rat forms about 5 per cent of the whole bodj', which 

 is about one-fourth larger relatively than in the pig at birth.* In the 

 adult rats the brain forms about . 7 per cent of the bodj', which is 

 eight times as large relatively as in the adult hog. 



In the .dog-fish data by Kellicott ('08), show that at birth the 

 brain forms 1.11 per cent of the total, while in the adult it forms 

 only 0.085 per cent or about the same relatively as in the pig. 



Some observations on the chick, recoi'ded by Welcker and 

 Brandt ('03), indicate that, at the ninth day of incubation, the 

 brain forms 28.2 per cent of the body; newly hatched, 3 per cent; 

 adult, less than 0.5 per cent. 



Spinal cord. {Table 1: fig. 3). The si:)inal cord in the pig 

 embryo has its maximum relative weight (of the stages observed) 



