1889.1 Embryology. 931 
ered by Nagel that the germinal epithelium in the mammalian embryo 
is much greater in extent than is required for the rudiments of either 
ovaries or testes. Similar facts are known respecting the development 
of the ovaries and testes of other forms. In the human female at 
puberty there are potentially 72,000 ova in the ovaries, yet of these 
only about 400 can by any possible chance ever become mature in a 
lifetime, while an average fertile marriage would, instead of increasing, 
actually reduce the number to 350. This is due to the interference of 
the process of gestation. So pronounced is this interference in its 
physiological effects that it leads to the development of a marked 
difference between the corpus luteum formed in the ovary in the 
pregnant and that formed during the non-pregnant condition. 
To sum up, the secondary processes of ova-gestation, viviparity, and 
utero-gestation have tended to diminish the fertility of a species as 
measured by the whole number of ova produced in a lifetime ; but at 
the same time a great gain was made in the strength, vigor, and oppor- 
tunity for survival of the offspring. Ova-gestation, all forms of vivi- 
parity, except the parthenogenetic, and utero-gestation, are probably 
the consequences, in the first place, of the acquisition of the ability to 
effect a fertile union or copulation of the sexes, the impulse toward 
which probably came originally from the male, where the sexes were 
These processes have also tended to direct nutriment from the ova 
and ovary to be built up into the embryo in other ways, thus tending 
to intensify the alecithal or yolkless condition of the other eggs re- 
maining in the ovary ; or where a brief ovo-gestation only occurs, as 
in birds, reptiles, and monotremes, the surplus nutriment has been 
concentrated upon the few serially matured ova, thus increasing the 
actual volume of the latter rather than diminishing it. If, however, 
prolonged utero-gestation supervenes, the opposite effect must follow, 
and nutriment be continually diverted from the ovary to the uterus, 
and thus tend to diminish the size of the ova remaining in the ovary. 
The subsequent period of lactation would tend to prolong this diver- 
sion of surplus nutriment from the ovary in mammals. 
The fertility of the marsupialia is much greater than that of other 
mammalia, and the eggs, as was to have been expected from the 
shorter period of gestation, are also much larger. I have myself 
removed twenty-two ripe ova from the uteri of a single female of the 
common Virginia opossum. Other facts with which I have become 
familiar in a study of the gestation of mice and rats, tend to show that 
an embryo may develop to a certain extent and then undergo histo- 
