44 
ON A PLATYPUS EMBRYO, 
laid as 1 5 mm. by 1 2 mm. Three other females were shot on the 
same date, and these had obviously just laid their eggs, as 
evidenced by the emptiness and large size of the left uterus and 
by the presence and condition of corpora lutea in the ovary. 
Moreover, the mammary glands in all four females were of 
approximately the same size. The tubules were arranged in a 
fan-like fashion, radiating outwards from the, at this stage, very 
small bare area of the ventral abdominal wall, and measured 5 cm. 
in length. 
From the size of these eggs as compared with Caldwell's, and 
from the condition of the other three females shot on the same 
date, we may reasonably conclude that they were just ready to be 
laid. 
The only other recorded measurements besides Caldwell's of the 
size of the Platypus eggs when laid are contained in a paper by 
Geoffroy St. Hilaire published in 1829.* The eggs, nine in 
number, were found lying on a rough nest in a small burrow on 
the banks of the River Hawkesbury, N.S.W., and measured 
If inches (34 mm.) long by J of an inch (19 mm.) broad. The 
eggs here described were probably not those of Platypus at all : 
as St. Hilaire himself afterwards pointed out, they could not on 
account of their size pass through the pelvis, and he then came to 
the conclusion that in Platypus the eggs must be hatched inside ! 
The eggs were opened immediately after the animal was shot 
and their contents preserved in picro-sulphuric acid. The embryos 
were stained with borax-carmine, imbedded in paraffin and cut 
into serial sections with the Cambridge microtome. 
To our friend, Prof. J. T. Wilson, we are indebted not only for 
many valuable suggestions but for much kindly criticism during 
the course of our work, and we desire here to tender him our 
sincere thanks. We have also to thank Messrs. Shewen and 
Grant, assistants in the Physiological Laboratory, for much assist- 
ance in the preparation of the photo-micrograph accompanying 
this paper. 
* Aim. ties Sc. Nat. T. xviii. p. 162. 
