SEXUAL CYCLE OF THE GUINEA PIG. 249 



third attempt during the next two periods of heat were mostly 

 successful. 



Sobotta 1 states that the guinea pig in captivity can become 

 pregnant more frequently in summer than in winter. My 

 observations do not bear out this statement. In my experience 

 the pregnancies resulted almost as often in the cold as in the 

 hot season. The -winter climate is better adapted for the guinea 

 pig in captivity than the hot summer weather when the tempera- 

 ture is liable to exceed 90 F. Under those conditions the animal 

 suffers and becomes poorly developed as the result of under- 

 nourishment. On the other hand, it is almost always in good 

 condition in winter. Therefore the cold season agrees better 

 with guinea pigs than the hot season under conditions of cap- 

 tivity. In exceptional cases the guinea pig may become in 

 winter time too fat to be favorable for impregnation; and this 

 may even happen in summer time. 



Relation between Growth, the First Period of Heat and the Period 

 of Sexual Maturity in the Guinea Pig. 



In my observations the weight of the guinea pig is ordinarily 

 three ounces soon after birth. However, the weight of the 

 young is in accordance with the age of the mother and also the 

 number of the litter. As a rule the young born from a young 

 mother are small in size and in litter. On the other hand, when 

 the mother is older than 5 months, the weight of the young is 

 mostly larger. For instance, in one litter the young weighed 

 6 or 7 ounces soon after birth. 



Minot 2 has concluded that the guinea pig is slow in growing 

 and that it attains its full size one year after birth. My observa- 

 tions are as follows: The young guinea pig at the time of the 

 first heat was about two months old; about 7 to 10 days before 

 the beginning of heat, . the growth almost stopped or at least 

 progressed very slowly. At the time of the first heat period the 

 average weight was n to 13 ounces. After the first heat period 

 had passed it began to grow very rapidly. About two or three 



1 Sobotta, Anal. Hefte, 1906, XXXII. 



2 Minot, C. S., "Growth and Senescence," Journ. Physiol., 1891, XII., 97. 

 "Problems of Age, Growth and Death," Popular Science Monthly, 1907, LXX.. 

 481, and LXXI., 97 and succeeding numbers. 



