Prenatal Growth of the Human Body. 147 



The only other, mammalian form upon which specific data con- 

 cerning prenatal growth are available (so far as I know) is the 

 guinea pig. A few observations by Hensen (20) show that, in gen- 

 eral, the relative growth is more rapid during the earlier stages (from 

 16th day) than in later fetuses. There are, however,, some irregulari- 

 ties and evidently great individual variations. 



Concerning the relative size and grow^th of the various organs and 

 parts in embryonic life, exact data are still scarce. Certain facts, 

 however, are already generally known, or can be easily observed from 

 specimens or published figures. In all vertebrates, from the fishes 

 upward, the embryonic head is relatively large, especially in the 

 earlier stages. There is considerable variation in the extent to which 

 this is true in different forms, however. It appears most strikingly 

 developed in the amniota ; less so, as a rule, in amphibia and" fishes. 

 The head is perhaps relatively largest in the embryos of birds, where 

 it may form more than half the entire body. Among mammals 

 there is also variation in different species ; e. g., the head of the pig 

 embryo is relatively much smaller than that of the rabbit or human 

 embryo. 



The extremities in all forms appear relatively small in the embryo, 

 gradually increasing to the relative size of the adult. 



Correlated with the size of the head, we find the brain always 

 relatively larger, in the vertebrate embryo. It is almost always 

 largest at a comparatively early stage, diminishing thereafter in 

 relative size throughout prenatal and postnatal life up to the adult 

 stage. In the chick, a few observations are recorded by Welcker and 

 Brandt (45) indicating that at the 9th day of incubation the brain 

 forms 28.2 per cent of the body; at the 10th day, about 14 per cent; 

 11th day, 13 per cent; 13th day, 9 per cent; 17th day, 5 per cent; 

 newly hatched 3 per cent; adult, less than .5 per cent. Similarly in 

 the dog, shrew, salamander and stickleback, observations indicate 



weight and span of life in man and white rat have been reduced to the 

 same basis. The actual growth rate in the guinea pig and rabbit has been 

 shown by Minot (33) to be about 25 times as great as in the human body. 

 Oppel (.37) points out that animals of greatly diverse adult size are much 

 less different in size at corresponding early embryonal stages. 



