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 l^y the emptiness and large size of the left uterus and 

 hj the jDresence 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 KSt. 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 

 1| inches (34 mm.) long by f 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 joreserved 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. 



* Ann. (les 8c. Nat. T. xviii. p. 162. 



