CYTOMORPHOSIS 29 



create the new extremity. Afterward they differentiate 

 themselves in part in order to form the various tissues which 

 are characteristic for the limbs of Crustacia, Fig. 17. The 

 nerves and probably the blood-vessels penetrate subsequently 

 into the newly formed extremity. To conclude: Until it is 

 shown in at least one case with absolute certainty that 

 regressive development occurs it must remain very improbable 

 in the minds of earnest biologists that such a development 

 occurs at aU, or can occur. 



Cytomorphosis defines comprehensively all structural 

 relations which cells or successive generations of cells undergo. 

 It includes the entire period from the undifferentiated stage 

 to the death of the cell. The differentiations which occur in 

 the body are very different among themselves, and as is 

 well known these differences are much greater in the higher 

 than in the lower animals. Hence it is by no means easy to 

 recognize at once what is common to these changes, but some 

 important results have already been won. First of aU it is 

 to be stated that the differentiation in all cases shows itself by 

 visible new functioning structures in the protoplasm. There 

 exists here between the protoplasm and the nucleus a marked 

 contrast, for, as you have learned, the nucleus acquires, strictly 

 speaking, no new structures, although it also changes with the 

 progressive development. 



We know that the visible alterations in protoplasm are 

 initiated by invisible ones. Various experiments afford the 

 proof of this. The first rudiment of the fore-leg of the 

 larva of an amphibian may be cut off and then grafted into 

 another part of the body, where the rudiment will develop 

 further. ^^ The rudiment, or anlage, at the stage which is 

 specially suited to this experiment, is a little bud on the 

 surface of the larva. Microscopic examination shows that 



