CYTOMORPHOSIS. 27 



they may be designated as somatic cells. Besides the process of differentiation 

 of the cells, we find that the production of organs is largely dependent upon the 

 unequal growth of the germ-layers, one part growing rapidly, another more 

 slowly, so that the layers acquire, as the embryo develops, a more or less com- 

 plicated form, owing to the folding of the layers. The general principles which 

 govern these important developments are considered in the section upon the 

 Relations of Surface to Mass. 



Cytomorphosis. 



This term is used to designate comprehensively all the structural modifica- 

 tions which cells or successive generations of cells may undergo, from the earliest 

 undifferentiated stage to their final destruction. It will be convenient, though 

 somewhat arbitrary, to distinguish four fundamental successive stages of cyto- 

 morphosis. These stages are (i) the undifferentiated stage; (2) the stage of 

 progressive differentiation, which itself often comprises many successive 

 stages; (3) the regressive stage or that during which degeneration' or necro- 

 biosis occurs; (4) the stage of the removal of^the dead material. 



In the various parts of the body we find these stages to succeed one another 

 at varying rates, and there are always to be found in every living body a consid- 

 erable number of cells which have only passed through a certain differentiation 

 and do not present any of the phenomena of degeneration or of death. On the 

 other hand, there are cells at every epoch of life after the embryonic period 

 which degenerate and die off, although the life of the individual is uninterrupted. 

 At any given moment the body consists of cells, which have made unequal 

 progress through the cytomorphic cycle. 



I. The Undifferentiated Stage. — A fertilized ovum is an undifferentiated 

 being, although it has a very complex organization. As it has only one nucleus, 

 there can be no variety of nuclei. The term "undifferentiated" therefore ap- 

 plies especially to the protoplasm which seems to have a uniform essential 

 structure throughout, although the masses and strands of protoplasm may 

 exhibit characteristic peculiarities, especially in relation to the distribution of 

 the yolk. In the adult tissues, on the contrary, we see that the protoplasm of 

 the cells of different kinds offers many varieties of structure visible with the 

 microscope. We may legitimately conclude that the absence of similar visible 

 peculiarities in the egg, by which one part may be distinguished from another, is 

 evidence of uniformity of structure throughout the egg. We have also direct 

 experimental proof that the egg is uniform throughout, or, to use a better phrase, 

 that the egg is isotropic. Pfliiger, in 1884, proved that the side of the frog's egg 

 which normally develops into the ventral surface of the embryo can be made to 



