214 
CHA8. H. o’dONOGHUE. 
any stimulus derived from a foetus.” The series of growth 
changes occurring in Dasyurus after ovulation, not succeeded 
by pregnancy, has already been described in the preceding 
section, and its practical identity with that of pregnancy has 
been pointed out. In this case it is perfectly obvious that 
the enlargement commences quite apart from fertilisation, 
and is continued when there is no fertilised ovum present to 
produce an internal secretion. 
Lane-Claypon and Starling (loc. cit.) further state that 
in their opinion the chief source of the hormone in the early 
stages of pregnancy may perhaps be located in the 
chorionic villi. This, however, does not apply to Dasyurus, 
for here we find that the most active growth-period occurs 
while the embryos are in the blastocyst stage, and that by far 
the greatest part of the enlargement, due to the actual increase 
of glandular tissue, has occurred before the embryo becomes 
attached to the uterus at all. It has been pointed out above 
that the increase in size after this time is to be explained 
largely by the mechanical increase in the size of the lumena 
of both alveoli and ducts. It may be urged as an objection 
that although there is no placenta present there is still the 
trophoblastic ectoderm of the embryo, and this might give 
rise to the hormone. Although this is possible here, it is 
quite impossible in the cases of milk secretion by virgins 
when no foetus is present. Further, it should be remembered 
that at puberty there is a marked growth in the mammary 
gland, though no product of conception or organ of attach- 
ment is present. Halban (19) also looks to the placenta as 
the point of origin of the hormone during pregnancy, but 
says that during puberty and menstruation the changes in 
the mammary glands are dependent on a chemical substance 
derived from the ovary, and again, that in unusual patho- 
logical cases (e.g. abnormal growths in the ovary) the ovary 
is able to work in a similar manner to the placenta. Thus it 
does not seem satisfactorj' to consider the placenta as the 
point of origin of the stimulus, nor the presence of a fertilised 
ovum as necessary for its production. We now have left the 
