1010] ANIMAL PRODUCTION. 4G9 



i 



continued Di jxh until the seventh month, when the decline became inure marked. 

 After the fifteenth month the average size was well under 4. 



The second litter produced by a female waa Larger than the others, averaging 

 7.7. The first Utter averaged 7.2. The third t<> seventh pregnancies resulted 



in litters somewhat below the first litter In size. Thereafter there was a 

 steady decline. In the author's opinion, the age of the mother is more im- 

 portanl than the serial number 01 the pregnancy in determining litter size. 

 The size of her first litter is considered u better index of a female's fertility 

 than the size of the second. Coefficients of variation for litter size at different 

 ages are given. Iii general they are inversely proportional to the average 

 litter size for a particular age. 



The sex ratio of the young was 106.1 males to 100 females. No relationship 

 was noted between the variation in the sex ratio and the age of the mother, 

 except a slight Indication that old mothers tended to produce a relatively high 

 proportion of females. 



The prolonged gestation period in suckling mice, W. B. Kikkiiam (Anat. 

 I\(<\, tl {1916), No. 2, pp. 81-fO). — In order to discover the causes of the pro- 

 longed gestation period found by Daniel (E. S. R., 28, p. 173) in female mice 

 that are suckling young, the author studied the post-partum ovulation and de- 

 velopment of ova in two sets of females, one allowed to nurse from 3 to 8 

 young during pregnancy, the other deprived of their litters immediately after 

 parturition. 



Lactating females generally fail to ovulate immediately after parturition. 

 In the case of those that do hecome pregnant immediately, the course of 

 development of the dividing eggs is the same as in oonlactatlng females as f al- 

 as the seventh day, at which time the egL r s have reached the hlastula sta.Lie. In 

 nonsuckling females on this day, the emhryos hecome implanted in the uterus 

 and continue their development rapidly. In suckling females, at least in the 

 ten that were killed and examined during this Investigation, the hlastula? 

 remain free in the lumen of the uterus from the sixth to the fourteenth day 

 without undergoing development. On the fourteenth day implantation takes 

 place and growth continues. The emhyros of nonsuckling females by this time 

 have the lenses of the eye well developed, future location of Legs and ribs 

 clearly indicated, hair and teeth folicles formed, and the blood cells showing 

 signs of hemoglobin. 



The further development of the embryos in lactating females is apparently 

 Irregular and generally very slow, so that it is impossible to reconcile the 

 available data with the known length of gestation in this class of female. 



On the postnatal growth of the body and of the central nervous system in 

 albino rats that are undersized at birth, Helen I>. King (Anat. Rec, 11 

 (/.''/<>'>. No. 2, pp. .'/1-52). — It is stated that as a rule a female rat in good physi- 

 cal condition produces only young of normal birth welghl and growth capacity 

 when the number in the litter is small or medium. Occasionally, however, 

 undersized individuals occur which, if they survive the first day of postnatal 

 life, grow rapidly for some time, but this acceleration is correlated with early 

 cessation of growth, so that the ultimate body weight and especially the weight 

 of the nervous system is below the Standard. The difference, however, is often 

 not sufficient to prevent the animals being classified as " normal " at ma- 

 turity. Data on the growth of three litters in the Wlstar Institute rat colony, 

 containing individuals of this type, are presented. 



If the litter is very large or the mother is not in good physical condition 

 during the gestation period, some of her young may be born with their growth 

 capacity so impaired that they remain permanently dwarfed. These are the 



