all individuality is at first contained in the general" 

 (I Sch Ilia 156). This pre-existence of individuality in 

 the general, though not of course in the sense of primitive 

 preformation, others might consider an argument against 

 studies of new formation. But Baer disputed this point of 

 view. 



Study of histological differentiation shows that new 

 structures form not at an empty place, but by transformation 

 of the previously existing simpler and homogeneous formations. 

 The modern embryologist could describe the phenomenon of 

 cartilage formation ir. the extremities, for example, as the 

 transformation by condensation of mesenchymal cells into 

 cartilage. The same also relates to morphological differen- 

 tiation, because "each organ is a changing part of the more 

 general organism." Baer illustrated this with the example 

 of respiration development. When the lungs develop as 

 outgrowths of the mucous tube, there is already an elevation 

 between them which, after stretching, forms the trachea. 



Another example concerns the development of the extrem- 

 ities. Their first appearance could be called a new formation; 

 however, the skin layer which is separated from the spinal 

 and abdominal plates in the region of the future extremities 

 already contains primitive structures which initiate the 

 formation of extremities. 



In opposition to Serres, who considered that organic 

 growth is represented by the union of initially isolated 

 elements, as with crystal development, Baer confirmed that 

 the formation of an organ, like the formation of the whole 

 embryo, involves a sense of preformation. "The absolute 

 beginning of the process," he asserted, "is always imper- 

 ceptible" (I Sch II If, 157 - 158). 



The development of the whole embryo and its organs 

 universally occurs, in Baer's opinion, from the center to the 

 periphery, but the universality of this idea seemed similar 

 to the constructions of Naturphilosophie which were in general 

 so foreign to his thinking. In his next scholium, Baer 

 objected to this centrifugal character of development. This 

 fourth scholium, with its two corollaries, is called "On the 

 scheme which vertebrate development follows." 



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