154 



WEIGHT, SIZE, AND AGE OF THE HUMAN EMBRYO. 



From an examination of this table it is at once apparent that the weight incre- 

 ment per millimeter progressively increases throughout the fetal period. In fetuses 

 under 60 mm. long the weight increase for each additional millimeter in length is 

 less than 1 gram. In fetuses between 70 and 80 mm. long there is an average 

 increase of 1 gram per millimeter. This becomes 2 grams per millimeter in fetuses 

 between 90 and 100 mm. long, and 4 grams per millimeter in fetuses between 130 

 and 140 mm. long. In fetuses about 200 mm. long the weight increase is 10 grams 

 per millimeter; 300 grams, 20 grams per millimeter; and at term it reaches 25 

 grams per millimeter. 



It is of interest to note that, whereas there is a progressive increase in the 

 actual weight increment throughout the whole fetal period, the contrary is true 

 for the percentage weight increment which progressively decreases. For example, 

 in fetuses 90 mm. long, weighing 50 grams, with each millimeter increase in length 

 there is an increase of 2 grams in weight, i. e., 4 per cent; in fetuses 160 mm. long, 

 weighing 298 grams, the weight increment is 6 grams per millimeter, i. e., 2 per 

 cent; in fetuses 300 mm. long the weight increment becomes less than 1 per cent. 



Table 2. — Weight increment per millimeter for fetuses of various sizes. 

 The increment given is the average fur the respective 10-millimeter intervals. 



RELATIVE VARIATION IN SITTING HEIGHT AND WEIGHT. 



It has been generally known that the length of the fetus exhibits less variation 

 than its weight and in this respect is regarded as a more accurate criterion of age. 

 From the data contained in our tables, and also from charts 1, 2, and 4, the normal 

 variation in length (always referring to sitting height) relative to weight can be 

 fairly accurately determined. Although it is true that the percentage variation 

 in weight is greater than that of sitting height, yet the weight increment for each 

 week is so much greater than that of length that age determination based on 

 weight has an accuracy nearly as great as that based on length. The combination 

 of the two increases proportionately the accuracy of age determination, and for 

 this reason charts 1, 2, and 4 should prove of practical value. 



At the fourteenth week the fetus has a mean sitting height of 88 mm. and a 

 mean weight of 44 grams. If the far outlying ones are omitted, it will be found that 

 specimens of this age measure between 85 and 92 mm. in length, a range of 7 mm., 

 thus showing a variation of 8 per cent in length. At the same time, fetuses 88 

 mm. long vary in weight from 38 to 52 grams, i. e., 14 grams or a variation of 31 



