122 C. M. Jackson. 



11 mm., 17 mm., and 31 mm.) were measured from sections, the 

 remainder by the direct method described (7.3 mm. by weighing). 



The data obtained from these specimens form the basis for the 

 figures given for the first three months in Table III. The volume of 

 the human ovum (the diameter being assumed to be .2 mm., as usually 

 stated) is about .000004 cc, which, asisuming the specific gravity to 

 be 1.0, corresponds to a weight of .000004 g. The 7.3 mm, embryo 

 (volume .026 cc.) was probably somewhat shrunken by the alcohol 

 and embedding process so that the volume of .041 cc. obtained from 

 the His model of a 7.5 mm. embryo is perhaps nearer the true size 

 at the end of the first month, corresponding to a weight of about .04 g. 



Thus we obtain for the relative monthly growth rate^ of the 1st 

 month, 9999; for the 2d month, 74; for the 3d month, 11. Fehling 

 (16), whose figures are often quoted, gave no estimate for the 1st 

 month, but (without reliable data) estimated the relative monthly 

 growth rate for the 2d, 3d, and 4th months at 3, 4, and 5 respect- 

 ively, the greatest relative growth being in the 4th month. My ob- 

 servations, however, prove beyond doubt the conclusion of Miihlmann 

 (30), recently emphasized by Minot (34), that the relative growth 

 in the human embryo is by far the greatest during the 1st month, 

 declining rapidly at first, then more slowly hroughout succeeding 

 months. 



Even the large number, 9999, representing an increase of nearly 

 one million per cent, is in reality too small for the relative growth of 

 the human embryo during the 1st month. For as a matter of fact, 

 not the entire ovum, but only a portion of it, actually goes to form 

 the embryo. The remainder is concerned with the formation of the 

 membranes, etc. Since it is not known what proportion of the ovum 

 goes for each of these purposes, the problem may be approached in 

 another way. Table Ilia shows the weight of embryo plus mem- 

 branes and enclosed fluids, at the end of the 1st, 2d and 3d months, 



*The relative growth rate is the ratio of the gain during a given period 

 to the weight at the beginning of the period, and is the most accurate 

 index of the rate of growth. It indicates the increase in a unit of weight 

 during the given unit of time. Thus while the total amount of gain in absolute 

 weight increases steadily for each prenatal month, the gain per gram of body 

 weight (as shown by the relative growth rate) is constantly decreasing. 



