156 EMBRYOLOGY. 



in important points assent to His's view. When, for example, 

 His (1874, p. 50) seeks to reduce the mechanics of form to the 

 simple problem of the form-changes in an unequally stretched 

 elastic plate, in my opinion he overlooks the fact that a plate com- 

 posed of cells, even if it possess elastic properties, is, nevertheless, a 

 much more complicated structure, and that the processes of folding 

 and evagination are primarily produced by the energy of the 

 growth of special groups of cells, and are therefore not to be com- 

 pared with the bendings and stretchings of elastic plates. As 

 PANDER has already emphatically stated, one is not to imagine in 

 the folding processes a lifeless membrane, but rather the folds are 

 themselves of organic derivation, called forth at the proper place by 

 a cell-multiplication at that place. For this reason, too, HAECKEL 

 in his polemic, " Ziele und Wege der .heutigen Entwicklungs- 

 geschichte," has attacked this method of treating embryology,, 

 introduced by His. 



That the morphological differentiation of the animal body primarily 

 rests upon a process of folding of epithelial lamellae, my brother and 

 I have endeavored, by means of an abundant series of observations,, 

 to demonstrate in a still more exhaustive manner than our pre- 

 decessors. In our " Studien zur Blattertheorie " we have, in the first 

 place, directed attention to the Coelenterates as the animal organisms 

 in which the principle of the formation of folds is most clearly 

 shown througkout the whole organisation, even into details; and, 

 secondly, we have endeavored to establish for Vertebrates that 

 organs like the body-cavity, chorda, and primitive segments, which 

 it was claimed arose by a separating and splitting of cell-layers,, 

 likewise come into existence through the typical process of foldings 

 and constriction. 



Finally we have endeavored to point out a physiological cause 

 for the unequal growth of a cell-membrane, and have found such in 

 the Calenterates in the unlike functional activity of its various 

 regions. Parts of a membrane will grow more rapidly and must 

 become infolded, when in consequence of their position they are 

 called upon to accomplish more than neighboring regions. 



In concluding this historical sketch attention should be called to 

 the fact that C. E. VON BAER, in the general discussion of embryo- 

 logical processes, was the first to distinguish clearly between the 

 events of morphological differentiation, which take place in the 

 beginning of development, and those of histological differentiation,, 

 which occur later. 



