Page:Popular Science Monthly Volume 71.djvu/107

generations, and at the present time a multiplication of cells is going on in every one of ns. It never entirely ceases as long as life continues. The development of the body, however, does not consist only of the growth and multiplication of cells, but also involves changes in the very nature of the cells, alterations in their structure. Cells in us are of many different sorts, but in early stages of development they are of few sorts. Moreover, in the early stages we find the cells all more or less alike. They do not differ from one another. Hence comes the technical term of differentiation, to designate the modifications which cells undergo with advancing age. At first cells are alike; in older individuals the cells have become of different sorts, they have been differentiated into various classes. This whole phenomenon of cell

Fig. 5. Three Transverse Sections through a Rabbit embryo of seven and one half Days, from series 622 of the Harvard Embryological Collection. A, section 247 across the anterior part of the germinal area. B. section 260 across the middle region of the germinal area. C, section 381, through the posterior part of the germinal area. Magnified 300 diameters.

change is comprehensively designated by the single word, cytomorphosis, which is derived from two Greek words meaning cell and form, respectively. A correct understanding of the conception cytomorphosis is an indispensable preliminary to any comprehension of the phenomena of