PERIOD OF GESTATION IN WHITE MICE 
869 
what factor would be capable of thus causing delay in ovulation? 
It is a belief that in human beings lactation may thus delay or 
even prevent ovulation. 
But, if lactation delay ovulation and thus increase the period, 
it may be asked — why should not the delay be of practicall}^ 
equal duration in all cases? Why should a mouse when suckling 
few young, go a relatively short time and when suckling more, 
go a longer period of time? Does this not suggest amount of lac- 
tation and does not amount suggest nutrition? 
If it could be shown that lactation, by robbing the developing 
young of nourishment, causes them to develop more slowly before 
birth, as malnutrition may retard development after birth, a 
second explanation for the delay, equally as effective as that of 
delay in ovulation, would be at hand. In this case the delay would 
be in the period between fertilization and birth and not in the act 
of fertilization. In either case lactation might be the cause of 
the delay. 
But whether the delay is in retarded ovulation, that is, before 
fertilization, or in nutrition after fertilization, is a problem open 
to experiment. Reserving a detailed discussion for a future com- 
munication on the subject, it may be said that two sources of evi- 
dence seem to show that, at least in mice, lactation does not delay 
ovulation. 
By microscopic examination Sobotta, in working over a great 
deal of material, found that ovulation and parturition occur close 
to the same time. Lams et Doorme confirm the opinion of 
Sobotta and add (p. 284), ^^En somme, la mis-bas, la rupture des 
follicules murs, le rut et Taccouplement se suivent ordinairement 
tres vite." Long, in a paper now in press, shows that ovulation 
takes place in from 14^ to 28J hours after parturition. From this 
source the common conclusion is that ovulation is not delayed. 
From experimental evidence which I now have it is strongly sug- 
gested that, in no way does lactation lengthen the period before 
fertilization. A case in point may be given in conclusion. A 
mouse, designated as ''F" which in examples 8 and 9, table 2, 
had two periods of lengthened gestation, gave birth to young 
and as is usual copulated the same day. At the end of the fourth 
