84 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
impregnated. We see here, after the lapse of perhaps two cen¬ 
turies, no loss in the power of growth. 
I would suggest that the reason why the animal ceases at last to 
grow is the same as the reason why the differentiated tissue below 
O •/ 
the tip of the epicotyl ceases to grow — not because there is a 
necessary limit to growth force at a certain distance from impregna¬ 
tion, but because it is advantageous to the species that the individual 
should cease to grow at this point. If it is not advantageous for it 
to cease to grow, it may go on for centuries like the tip of the plant. 
To sum up, then, my studies lead me to recognize a general 
parallelism between the developmental processes occurring at the 
tip of a twig and in the animal embryo. In both there is first a 
period of rapid cell division with slow growth; next, a grand period 
of growth in which the general form of the embryo is acquired, the 
Anlagen of the organs are established, and the organism increases 
rapidly in size by imbibition of water; and, lastly, a period in which 
histological differentiation is carried on while the absolute growth 
increments cease to increase. Finally, the fact that increase in body 
substance is so largely due to a non-growing substance — water — 
diminishes the value of the percentage increment curve of growth. 
Printed , June , 1897. 
