period between 28 and 36 hours essential changes in 

 embryonic development take place C$ 167) . 



The increase of size and the progressive formation shows 

 that the embryo feeds on the egg substance, From here it 

 follows, according to Wolff, that the nutritional particles 

 pass from the egg to the embryo. And all of this is accom- 

 plished by means of a power. As this power is not the 

 contraction of the heart and arteries or the muscles, 

 since all these parts do not yet function or do not yet 

 exist, and since the power acting here sends away the nutri- 

 tional substances by a canal, then it, is consequently 

 analogous to the essential power ($ 168) . Wolff considered 

 that the nutritional juices in the embryo of the chicken 

 move, as in plants, stimulated by the essential power, through 

 the substance built from globules and deposited between 

 these globules, and by this movement they increase the size 

 of the embryo ($ 169) . 



Animal substance, as plant substance, is characterized 

 by the ability to solidify. This substance is soluble in 

 water. While standing and during simultaneous slight heating, 

 it gradually loses water and is transformed at the beginning 

 into a more or less thick, and then hard body C$ 171) . Wolff 

 considered that the ability of solidification of the animal 

 substance is expressed in a lesser degree than that of the 

 plant; its solidification occurs more slowly and it never 

 reaches that degree which is characteristic for the plant 

 substance. He confirmed this for the cellular tissue of 

 animals, which never possesses that compactness and solidity 

 possessed by the wood of even young trees (§ 172) . 



Returning to his description of phenomena, Wolff turned 

 attention to the moment which precedes the beginning of 

 heart pulsation. In the umbilical vascular area (area 

 vasculosa), the substance of the embryonic disc disintegrates 

 into islets. Their number progressively increases, and their 

 color changes from white to dark-yellow to red. It follows 

 from that, that in other eggs practically in the same stage of 

 development, where the fluid in the islets is slightly more 

 red, the heart pulsates extremely animatedly ($ 179) . This 

 description, stated nearly in Wolff's actual words, shows that 

 he distinctly saw the formation and development of blood islets 



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