According to His[104], the following are the mean lengths of the unborn human embryo, from month to month.

Months012345678910 (Birth)
Length in mm.07·54084162275352402443472490–500
Increment per month in mm.—7·532·544781137750412918–28

Fig. 8. Mean monthly increments of length or stature of child (in cms.).

These data link on very well to those of Rüssow, which we have just considered, and (though His’s measurements for the pre-natal months are more detailed than are those of Rüssow for the first year of post-natal life) we may draw a continuous curve of growth (Fig. [7]) and curve of acceleration of growth (Fig. [8]) for the combined periods. It will at once be seen that there is a “point of inflection” somewhere about the fifth month of intra-uterine life[105]: up to that date growth proceeds with a continually increasing {76} velocity; but after that date, though growth is still rapid, its velocity tends to fall away. There is a slight break between our two separate sets of statistics at the date of birth, while this is the very epoch regarding which we should particularly like to have precise and continuous information. Undoubtedly there is a certain slight arrest of growth, or diminution of the rate of growth, about the epoch of birth: the sudden change in the {77} method of nutrition has its inevitable effect; but this slight temporary set-back is immediately followed by a secondary, and temporary, acceleration.

Fig. 9. Curve of pre-natal growth (length or stature) of child; and cor­re­spon­ding curve of mean monthly increments (mm.).

It is worth our while to draw a separate curve to illustrate on a larger scale His’s careful data for the ten months of pre-natal life (Fig. [9]). We see that this curve of growth is a beautifully regular one, and is nearly symmetrical on either side of that point of inflection of which we have already spoken; it is a curve for which we might well hope to find a simple math­e­mat­i­cal expression. The acceleration-curve shown in Fig. [9] together with the pre-natal curve of growth, is not taken directly from His’s recorded data, but is derived from the tangents drawn to a smoothed curve, cor­re­spon­ding as nearly as possible to the actual curve of growth: the rise to a maximal velocity about the fifth month and the subsequent gradual fall are now demonstrated even more clearly than before. In Fig. [10], which is a curve of growth of the bamboo[106], we see (so far as it goes) the same essential features, {78} the slow beginning, the rapid increase of velocity, the point of inflection, and the subsequent slow negative acceleration[107].

Fig. 10. Curve of growth of bamboo (from Ostwald, after Kraus).