THE CELLULAR CHANGES OF AGE 45 



bouring nuclei. All the primitive parts are then 

 true syncytia. Thus it happens that in Fig. 9, which 

 represents sections of a very young rabbit germ, the 

 single cells are not marked off. Nevertheless it is 

 customary and convenient to speak of the cells even 

 at such a stage. The actual delimitation of the cells 

 occurs in older stages in nearly every part of the 

 body. The blood corpuscles are always the first cells 

 in vertebrates to become definitely individualised. 



There is, in fact, as you probably all know, a con- 

 stant growth of cells ; and this growth implies also, 

 naturally, their multiplication. There has been in 

 each of us an immense number of successive cell 

 generations, and at the present time a multiplication 

 of cells is going on in every one of us. It never 

 entirely ceases as long as life continues. The der 

 velopment 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 earliest 

 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. Some authorities use specification as a tech- 

 nical synonym for differentiation. At first cells are 

 alike ; in older individuals the cells have become of 

 different sorts, they have been differentiated into 



