32 BIOLOGY 



in the percentage of water in the brain is determined for the 

 two forms at the equivalent ages, these two growth changes are 

 found to be nearly the same in both — Donaldson ('18). 



For the determination of equivalence in age among mammals 

 Brody and Ragsdale ('22) have used the three cycles of growth — 

 termed respectively the infantile, juvenile and adolescent cy- 

 cles and have taken conception rather than birth as the point 

 of departure. When by this method the conceptional age of the 

 maximum of the third (adolescent) cycle is multiplied by the 

 constant 13, it gives 36.5 months, or approximately three years 

 as the span of life for the rat. This is in good agreement with 

 our determinations as just given. The procedure does not yield 

 satisfactory results in the case of man. 



The menopause occurs at 15-18 months, which is equivalent 

 to 38-45 years. 



Functional similarities. Like man the rat is omnivorous and 

 capable of adapting itself to a wide range of diets. 



Rate of response. Owing to the fact that the rat is living 

 thirty times as fast as man, its responses to all the changes in 

 its environment are relatively very rapid. To food, drugs, and 

 alterations in external conditions, it reacts with remarkable 

 promptness — and while this is very favorable for some sorts 

 of study, it makes it difficult to maintain the animal in a con- 

 stant condition for any considerable length of time. 



BIOLOGY : REFERENCES 



Life history. Donaldson, '06. King, '13, '24. Kirkham, '10. Kirkham and 

 Burr, '13. Lane, '17. Namiye, '09. Slonaker, '07, '12. Stewart, 1898. Wada, 

 '23. Watson, '03. Wang, '23. 



Span of life. Brody and Ragsdale, '22. Donaldson, '06. Mitchell, '11. 

 Slonaker, '12, '12 a. 



Puberty, ovulation, oestrous cycle and meno-pause. Greenman and Duhring, 

 '23. Kirkham, '10. Kirkham and Burr, '13. Long and Evans, '22. Sobotta 

 and Burckhard, '10. 



Period of gestation. Long and Evans, '22. 



Lengthening of gestation period. Greenman and Duhring, '23. King, '13. 



Superfecundation and superf elation. King, '13. 



Parturition. Hammett, '18. Todd, '23. 



Fertility. Crampe, '84. Evans and Bishop, '23 a. Greenman and Duhring, 

 '23. King, '15, '24. King and Stotsenburg, '15. Kolazy, 1871. Lloyd, '09 a. 



