274 BIOLOGICAL LECTURES. 



responds to the stem of the plant ; the blood vessels of the 

 vascular area that bring the nutriment from the yolk to the 

 embryo are the roots ; the organs and appendages of the embryo 

 correspond to the branches of the plant. 1 The organism starts 

 out on its development with a stem which is to connect it 

 with the source of nutrition on the one hand and its branches 

 on the other. All the substance of the embryo is originally 

 unorganized, inorganic. Organization sets in at one point in 

 the stem and thence gradually spreads to the tips of the 

 branches. A branch is first formed as a little bud of unor- 

 ganized substance, then the sap (in plants) or the blood (in 

 animals) flows into it from the adjacent organized part ; thus 

 it becomes organized, and the process continues till the organism 

 has acquired its definitive size and development. The blood or 

 sap is propelled into the unorganized substance, consisting of 

 globules, by a peculiar force Wolff's vis essentialis, which is 

 defined in the opening chapter of the Tkeoria, and was made 

 the special subject of Wolff's last work, written thirty years 

 later. 2 Organization of the unorganized substance is the com- 

 bined result of this vis essentialis and a property which Wolff 

 calls solidescibilitaSy a tendency to solidify, shown most clearly 

 in the formation of the walls of plant cells. The vis essentialis 

 propels liquid nutriment into the dense unorganized matter 

 already present. The paths along which it flows become the 

 cavities of the blood vessels or plant vessels not before existent. 

 The liquid nutriment solidifies to form more unorganized sub- 

 stance, by intussusception with that already present, and the 

 part grows. Wolff explains the origin of the kidney which he 

 discovered in the chick the Wolffian body, or mesonephros, 

 as we now call it in a similar manner. Here it is the urine 

 that is impelled by the vis essentialis into a mass of preexist- 



1 Wolff (Theorie von der Generation, 1764) compares four-footed animals to 

 pinnatifid leaves and " the bat is a perfect leaf a startling statement, but, as 

 I have shown, the analogy is not chimerical, for the mode of origin of the two is 

 the same" Quoted by Huxley ("The Cell Theory," Brit, and Foreign Medico- 

 Chir. Review, vol. xii, 1853). 



2 Wolff, C. F. Von der eigenthiimlichen und wesentlichen Kraft der vege- 

 tabilischen sowohl als auch der animalischen Substanz. St. Petersburg, 

 1789. 



