248 O. ISHII. 



that if a mother suckled three young, the growth of the latter was 

 very slow, or death resulted; in addition the weakening effect 

 on the mother was quite noticeable; it sometimes caused her 

 death, and in most cases the death of the young occurred. If 

 the young died, the mother soon regained her health and strength. 

 The result will therefore be disappointing either as far as the 

 condition of the mother or of the young is concerned. It has 

 been my experience that when the young were taken away from 

 the mother within 15 or 20 days after birth and the ordinary 

 vegetable and oat diet was given them, development was very 

 slow and in most cases death occurred. In any case the young 

 to attain healthy growth should not be taken from the mother 

 until more than 25 to 30 days of age. Under natural conditions 

 they cease suckling after 35 to 40 days, when they seek their own 

 food. I am therefore of the opinion that a litter of three very 

 seldom shows normal development when suckled by one mother. 

 When I combined one mother with two and another with 

 three young, each mother suckled on the average 2^/2 young. 

 In such cases I obtained good results. While two or three young 

 is the average size of a litter, I have known the number to vary 

 from one to six; the latter number, however, only occurred once 

 in my observation. When but one young is born, it is usually 

 well developed and vigorous; but to be successfully raised it 

 must be suckled for the usual period or at least for a time ap- 

 proaching it. There is another symptom in addition to the 

 bodily weakness, which the mother shows as the result of too 

 intensive suckling: it consists in the changing of the color of 

 the pupil of the eye which becomes pale or white. It may be 

 associated with general exhaustion. 



Under what Condition is Copulation followed by Pregnancy? 



As we stated above the normal duration of heat is about 10 

 hours. If copulation took place within 3 or 4 hours after the 

 beginning of heat, pregnancy followed only rarely. If the copula- 

 tion took place at a later period of the heat, better results were 

 obtained. According to my observations pregnancy occurred 

 in about 73 per cent, to 80 per cent, of the cases. In such cases 

 in which copulation was not followed by pregnancy a second or 



