84. A. A. W. HUBRECHT. 
never less than two are present, occupying the right and the 
left half of the uterus. 
There, as formerly in Sorex, I have, however, been able to 
establish without doubt that the number of fecundated eggs 
and even yet of early blastocysts is constantly found 
to be more considerable than the number of ripe 
foetus that attain maturity and form the normal contents of a 
litter. 
Thus in Tupaja four and sometimes more blastocysts are 
found in early stages, apparently all of them in equal conditions 
of vitality. A struggle between these blastocysts for the 
definite attachment to the maternal uterine wall is thus 
inevitable. How this struggle is brought about and what 
points finally decide between those that shall thrive and those 
that shall perish is at present obscure. Still the fact has no 
doubt a definite significance, considering that it is not a casual 
observation, but a most regular occurrence in at least two genera 
of Insectivora. 
My preparations are not yet numerous enough to allow me to 
speak with the same emphasis for the other genera. 
Attention will of course have to be directed to this point, in 
order to make out whether it may be regarded as a general rule 
in mammals that more blastocysts than can partake in the 
normal course of intra-uterine development are present in the 
earliest days after fecundation has taken place. 
One question to which my preparations do not allow me to 
reply is that concerning the duration of pregnancy in the five 
species investigated. ‘The lapse of time that occurs between 
the date of fecundation and that of parturition is in no way 
indicated even by the most complete set of intermediate stages 
between the cleaving egg and the ripe foetus. On the other 
hand, it is in no way of any importance for the correct interpre- 
tation of the different and successive ontogenetical processes to 
be acquainted with the exact rate at which these stages succeed 
one another, or with the age of any particular stage as expressed 
in a fixed number of days. 
With animals bred in domesticity this is of course easily 
