PRESIDENTIAL ADDRESS. 51 
In #. norvegicus, 9 followed by 7, and then 5 followed 
by 8, was the order of magnitude in 240 pregnancies. It is 
rather inexplicable to find that the number for 6 is less than 
half that for 5; whilst the numbers for 7 and 9 are more 
than for 5. Also that the number for 8 is slightly less than 
that for 5; whilst the numbers for 7 and 9 much exceed 8. 
I ean offer no explanation for this difference, but had odd 
and not even numbers been the lower, this might have sug- 
gested that each ovary supplied usually the same number of 
ova for fertilisation. 71.67 of the 240 pregnancies showed 
5 to 9 feetuses present. 3 
The presence of a small number of feetuses, say 1 or 2, 
can be explained by obstacles to fertilisation. An interest- 
ing feature in the two rats, one absent in the mouse, is the 
occasional occurrence of a number of fetuses much more 
than double that usually found. Perhaps these may be in- 
stances of superfetation or of the maturing of a double 
number of ova. Otherwise they must be considered as mu- 
tations. 
These New South Wales figures for the numbers of 
foetuses found in pregnant EF. rattws compare remarkably 
closely with some that I have published! for Western Aus- 
tralia. In 221 pregnant rats of this species examined in 18 
months, 1143 fcetuses were found, giving an average of 6.43 
per pregnancy, the figures for New South Wales being 6.66. 
Whilst examining the pregnant rats in Perth, I was 
struck by the fact that occasionally one horn of the bi- 
cornuate uterus might contain more fetuses than the other. 
This led to making notes, the substance of which was pub- 
lished in the above Bulletin, to see how great the divergence 
might be, and as to whether one side or the other tended on 
an average to have a greater number of young. 
2Bull. of the Dept. of State Med. and Pub. Health, W.A., Nos. 
10-12, 1909, p. 16. 
