ova of the Opossum tribe. 237 
The mode of formation of the ova in the ornithorhynchi, 
constitutes the intermediate link between that of the American 
opossum and the bird. 
The yelk-bags in the ovaria of the ornithorhynchi are more 
distinct, and less deeply imbedded than in the opossum : there 
isrno regular uterus, nor Fallopian tubes ; the yelk-bags pass 
along an oviduct, the lower part of which performs the 
office of a uterus. In this situation the ova are impregnated ; 
the penis of the male, which is bifid, throwing the semen 
into both oviducts at the same time, through several points 
like a watering pot, so as to scatter it all over the cavity. 
The ova are aerated by the vagina. The ova in a magni- 
fied state are represented in the annexed drawing. The 
organs of generation are figured in the 92nd volume, Plate 
IV. of the Philosophical Transactions. To show that the 
yelk-bags in the ornithorhynchus resemble those of the 
pullet, a magnified drawing of them made by Mr. Bauer in 
that bird, is annexed. 
To those members not conversant in comparative ana- 
tomy, the following summary may be acceptable. 
In the human species, and quadrupeds in general, the ova 
are formed in corpora lutea, and pass into the uterus, to the 
sides of which they become attached ; when the foetus is com- 
pletely formed it is expelled by the vagina, and afterwards 
sucks the mother. 
In the kangaroo the ova are formed in corpora lutea, re- 
ceive their yelks in the Fallopian tube, and their albumen 
in the uterus. The ovum thus completed, is impregnated in 
the uterus, aerated by means of lateral tubes, and when 
the young is expelled from the uterus, it is received into the 
marsupium, and attached to the nipple of the mother. 
