PROLOGUE IX 



for the species and constant within the Umits of biological varia- 

 tion. Hence variations in the weight of this system must depend 

 mainly on the size of the constituent cells. 



In other systems, characterized by the epithelial, glandular 

 and connective tissues, there is probably much more latitude 

 in the number of elements produced, but withal a concomitant 

 variation in cell size as well. 



In the growing rat there is at birth a phase in which cell 

 multiplication is relatively active, though accompanied by the 

 enlargement of those cells already formed. 



In the nervous system cell formation comes to an end early 

 and in the muscles it is slow after thirty days and diminishes 

 with age probably in some of the other systems, although it 

 persists throughout life in the epithelia. On a combination of 

 these two processes of cell formation and cell enlargement de- 

 pends the growth of the animal as a whole, accompanied, of 

 course, by the accumulation of more or less inert material. 



The overgrowth, such as was instanced earlier, appears to 

 depend mainly on the precocious enlargement of the formed 

 elements without a corresponding differentiation, which, if it 

 occurred, would be accompanied by a relative change in the 

 percentage of water and the chemical status of the tissues; 

 fcince these changes are more closely correlated with the age 

 than with the size of the rat. 



While all this is in general true, nevertheless a shifting back 

 of puberty in the female, which has been observed, indicates a 

 real precocity of development in the ovaries and the associated 

 structures. In this instance the chemical changes are also 

 precocious. Thus age and size may vary widely, also age and 

 the incidence of puberty, and hence in this latter case age and 

 the phase of chemical development. 



These statements indicate that there are fluctuations in char- 

 acters both on age and on body weight, and also that there may 

 be a drift or trend in the form and functions of the albino rat 

 as domestication progresses. 



What then is the relation of the data and reference tables to 

 this state of affairs? 



