52 J. B. CLELAND. 
The following extremes were noted, the right and left 
figures referring to the respective cornua:—9-0, 6-1, 1-5, 
0-5, 7-2, 1-5, 5-1, 1-5, 3-8. In 101 rats 331 fetuses were found 
on the right side, and 326 on the left. It therefore appears. 
that either horn may have a considerable preponderance of 
foetuses, and that the average number on each side over a 
series iS approximately equal. In other words, it would 
appear that when a discrepancy exists, this is due to some 
accident, either mere chance or some pathological obstruc- 
tive process, and is not due to any inherent specific factor 
in the species, by which more ova were liberated into and 
fertilised in one horn than the other. 
From data, compiled by me in Perth between 1906-1908, 
over a period of 2% years, as to the percentage of pregnant 
females (of E. rattus almost entirely) to total adult females 
for each month, a definite variation seems to exist, if the 
totals of rats examined, which varied from 265 to 684, were 
sufficiently large to exclude undue error. In March this 
percentage was 14.7. From April to May, it fell from 
10.1 to 9. It rose to 15.7 in July, and 20.3 in August, to 
fall in September to 13.6. From October to February it 
ranged between 21.2 and 35.3. The young rats from the 
October to February pregnancies would reach maturity at 
about the plague period of the year. 
The Rats that Travel by Sea. 
As the old English black rat (Hpimys rattus), including 
the Alexandrine variety (EH. rattus alexandrinus), the Nor- 
way rat (Epimys norvegicus |decumanus|), and the com- 
mon house mouse (Mus musculus) are all subject to plague, 
it is of considerable interest to see which of these species 
is most prone to travel by sea. The most frequent traveller 
of the three would naturally be looked on, other factors. 
being equal, as the most likely introducer of the plague 
bacillus into unaffected parts. 
