DAVENPORT : THE ROLE OF WATER IN GROWTH. 79 
The originally spherical body—the egg — does not merely grow to 
a larger sphere but it grows faster in one axis than in the others and 
so becomes elongated; its trunk grows in cross-section faster than 
its tail; from the side of the gill-bars rapid growth occurs forming 
the gills ; at four points on the side of the body excessive growth 
takes place in skin, musculature, and mesenchyma, resulting in limbs. 
Meanwhile the amount of dry substance in the embryo remains 
constant; we have to conclude, therefore, that all these local 
growths are due to local imbibition of water. The question, What 
determines excessive local growths ? resolves itself into this, What 
determines excessive local imbibition of water ? What determines 
that water shall be taken in especially where the limbs bud out, or 
the gill filaments arise? The problem of differential growth — 
Fig. 5. Curve of length of shell of Lymnaea stagnalis at intervals from hatching up to 
85 days. From Semper Animal Life, p. 163. 
