CHAPTER VI 

 FORMATION OF THE GERM-LAYERS 



THE period that we are now about to examine is marked by 

 extensive movements of parts of the segmented egg as a result 

 of which the organs are formed. During the segmentation- 

 period the cells retain, as we have seen, the position in which 

 they arise, but with the appearance of the blastopore a new 

 period is initiated in which extensive movements of cells and 

 groups of cells take place. 



His's EXPERIMENTS WITH ELASTIC PLATES 



His ('94), from his studies of the behavior of elastic plates, 

 has concluded that many of the phenomena of the develop- 

 ing embryo are the mechanical result of the tensions set up 

 in the different layers. In the embryo the shoving, compres- 

 sion, or extension is supposed to result from the unequal growth 

 of different parts. When a cell-plate lifts itself up into a fold, 

 as a result of more rapid growth in that region than elsewhere, 

 there is present on the concave side a positive tension ( Druck- 

 spannung") and on the convex side a negative tension. Under 

 these conditions the cells become conical, i.e. they are small 011 

 the concave side and broad 011 the convex side of the fold. 

 Each embryonic cell tends of itself to become spherical and only 

 the surrounding conditions, resulting from the growth of sur- 

 rounding parts, determine the shape of each cell at any period 

 of development. His has tried to explain many of the changes 

 taking place in the early embryo as the result of this simple 

 folding principle. The inrolling of the medullary plate, the 

 formation of the eye-outgrowth from this plate, the formation 

 of the mouth-cavity and the gill-slit-folds, etc., are examples of 

 some of these changes. His pointed out how closely the forms 



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