202 



CERTAIN LAWS OF VARIATION. 



values, the times required by the embryo at the various 

 stages of development in order to double its length have 

 been calculated: 



From this table we see that the rate of growth of the 

 human embryo diminishes steadily, with one slight ex- 

 ception, from the earliest to the latest stages. So 

 enormous is the diminution in the growth that in the 

 last month of pregnancy it is only about an eightieth 

 part of that in the third week. In the still earlier 

 stages it seems valid to assume that the relative rate of 

 growth increases at the same proportionate rate as in 

 the later ones; and, arguing from the more or less 

 known volumes of the ovum at the time of concep- 

 tion, and of the embryo in its third week of growth, it 

 has been roughly calculated that the first doubling in 

 diameter of the fertilised ovum requires less than three 

 days, or about 1-2 00th part of the time in the last 

 month of pregnancy. 



With regard to post-embryonic growth rate, but little 

 need be said. A large number of observations have 

 been made on the length of children at birth, and the 

 mean of all the values for both sexes is approximately 

 50.0 cm. From Bowditch's tables of the statures of 

 Boston school children,* it is found that the mean 

 * Report of Massachusetts State Board of Health, p. 275, 1877. 



