CHAPTER 1 

 BIOLOGY 



1. Life history. 2. Span of life. 3. Puberty — ovulation — menopause. 4. 

 Period of gestation — lengthening of the gestation period. 5. Superfecundation 

 — Superfetation. 6. Fecundity and weight at birth. 7. Recognition of sex. 8. 

 Sex ratio. 9. Body weight according to sex. 10. Behavior, a) Under natural 

 conditions, b) Under experimental conditions. 



1. Life history. The albino rat is born blind, hairless, with 

 a short tail, closed ears and undeveloped limbs. It responds to 

 contacts and olfactory and taste stimuli, utters a squeaking 

 sound and is capable of some locomotory movements which are 

 a combination of wriggling and paddling. The head is always 

 searching. The young can find their way back to the mother at 

 about ten days of age (Watson, '03) . The eyes open at from the 

 14th to the 17th days, most often on the 15th or 16th. King has 

 also observed that in a given Utter the eyes of the females usu- 

 ally open some hours before those of the males. For some seven 

 days more, i.e., up to the time when the young rats are 21-22 

 days of age, they are dependent on the mother. After this they 

 may be weaned, although if permitted, the young will depend 

 partly on the mother for some days longer. 



This adjustment of relations fits with the fact that the female 

 may be impregnated one or two days after casting a Utter (Kirk- 

 ham, '10; Kirkham and Burr, '13) and since the gestation period 

 is about 21.5-22.5 days, this would enable the female to free 

 herself from the first Utter before the second one was born. As 

 will be pointed out later, the gestation period may be prolonged 

 in nursing animals. 



When the young rats become habituated to independence, i.e., 

 at about 25 days, they enter on a period of activity, the phases 

 of which have been followed by Slonaker ('07, '12). In the cases 

 which he observed, it was found that increasing age was accom- 



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