and the blood in them prior to the formation of the heart and 

 to the beginning of its activity. 



"Of the number of my most fortunate observations," Wolff 

 wrote in a commentary to this paragraph, "I relate that which 

 I am reprinting in Figures 7 and 8." I suppose that here it 

 is possible to overhear nature when it is engaged in very 

 important work, namely to see the transformation of fluid, 

 stealing between the islets, into blood." The observation of 

 the blood movement in the vascular area creates the impression 

 that the vessels already existed (Fig. 3, 7). Examining this 

 region under the microscope, Wolff did not see anything, 

 however, except distintegrated heaps similar to those 

 represented in Fig. 3, 4 aa. At this moment the true vessels 

 are still not formed. Somewhat later on, head of the embryo 

 and the first outlines of the brain, cerebellum and eyes are 

 outlined, and "the rudiments of the vertebral column in the 

 form of a chamber, filled with a homogenous substance are 

 visible; but the rudiments of the extremities have not yet 

 appeared" (Fig. 3, 10). At this point "the outlined cavities, 

 filled with blood bodies are clearly visible" (§ 180) . The 

 next step in the development of the vessels is the widening 

 of their cavities at the expense of the increasing amount 

 of blood and the increased thickening of the vascular walls; 

 the globules making up the wall of the vessel appear more 

 compressed. The most compact structure is possessed by the 

 part of the wall near its opening; the farther from the axis 

 of the vessel, the more the substances become softer and, 

 finally, are transformed into the ordinary loose cellular 

 tissue, filling the spaces between the vessels (§ 181) . 



The processes of development of the chick embryos are 

 distinctly connected with heat, obtained during incubation; 

 if giving heat is discontinued, then all formative processes 

 stop. Wolff considered that heat cannot be one of the specific 

 causes of organic development, however, because "all physical 

 changes in the world, to a certain extent ., are in need of heat, 

 as to a resolving power. Without heat all will be turned into 

 lifeless chaos" (§ 183) . Wolff saw the role of heat in 

 embryonic development as leading to the dissolution of yolk, 



6. Here and later on, in the present chapter the references 

 to figure are related to Table 11 of Wolff, reprinted in 

 Figure 3 . 



56 



