which then can be assimilated by the embryo (§ 186) . From 

 these considerations this conclusion follows: "We do not know 

 any other power, except heat and essential power, which shares 

 in the formative processes... Therefore there is no basis for 

 doubt that the supply of nutritional substances to the fetus., 

 takes place under the influence of the essential power" 

 ($ 187). 



The subsequent statement was dedicated to the detailed 

 description of how the vascular branches originate and the 

 difference in formation of the veins and arteries; later 

 Wolff addressed the development of vessels, nourishing of 

 vessels, venous valves and anastomosis (§ 215 and 216). 

 It is impossible not to agree with Wolff in these special 

 observations, because in the discussion above, his method 

 of embryological investigations and his fundamental opinions 

 about developmental regularity have already become clear. In 

 the conclusion, he compared development in plants and in 

 animals; then he concluded that here and there the same 

 formative principle acts. Any differences are expressed by 

 the following: animals have a heart, which is absent in 

 plants, and the latter have a central core and point of 

 growth, which are absent in animals; but these differences 

 do *not constitute the main point of plant and animal organisms 

 The peculiarities of animals and plants are the expression 

 of their form and structure only to the extent that these 

 are machines. The essence of the animal and plant depends not 

 so much upon the structure, as upon the activity of the 

 essential power (§ 216) . 



Then Wolff turned to the formation of organs, beginning 

 from the development of the extremities. After 36 hours of 

 incubation, even the rudiments of the extremities are absent 

 (Fig. 3, 5). In this stage the outlines of the eyes, brain, 

 cerebellum, medulla oblongata and spinal chord are visible in 

 the vertebrae; only the spinal vertebrae are clear, while 

 the lumbar vertebrae are slightly outlined. The periphery 

 of the embryo is occupied by a "light substance," which 

 Wolff called cellular and which he saw as composed of 

 globules ($ 218). This substance gradually combines from 

 both sides of the embryo in two definite points — on the level 

 of the lumbar vertebrae and on the level of the heart. In 

 these places prominences form (Fig. 3, 11), at the end of 

 which the extremities develop. These prominences "truly are 

 the first rudiments of the extremities" ($ 219) . 



57 



