CYTOMORPHOSIS. 11 



The primitive germ-layers consist of very simple cells, and are themselves 

 at first extremely simple in their organization. The majority of the cells which 

 they contain undergo a greater or less degree of modification as development 

 progresses. This modification is termed differentiation, and is more fully con- 

 sidered in our next section, on Cytomorphosis. It is probable, however, that a 

 certain number of the cells very early in the development are set apart, preserving 

 the primitive character of their protoplasm and taking no share in the formation 

 of the tissues of the body. These cells, comparatively unmodified, are known as 

 the germ-cells; compare page 25 and the section on Heredity. As the remaining 

 cells form part of the body of the individual, 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 complicated 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 necrobiosis^ 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 vertebrate 

 body a considerable number of cells which have passed through only 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 an early brief em- 

 bryonic 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 applies es- 

 pecially to the protoplasm, which never has any special structures or formed 

 parts, such as occur in the tissues and cells of the adult. It is, however, not uni- 

 form, but in many ova has distinct regional differences, which so far as hitherto 

 noted depend upon peculiarities in the masses and strands of protoplasm, and 

 upon the distribution of the yolk granules, of which there may be several kinds. 



