Mr F. E. Beddard on the Ovary of Echidna. 361 



I was able to examine a complete (or nearly complete) 

 series through an ovum of this size, and I failed to observe 

 any trace of an egg-membrane or follicular epithelium. As 

 shown in the figure, the ovum appears to be contained within 

 a capsule formed of a specially thickened layer of the ovarian 

 stroma. It does not, however, seem likely that both the 

 vitelline membrane and the follicular epithelium would dis- 

 appear in this way; and, unfortunately, I could not examine 

 any ova intermediate in size between this stage and ova of 

 1 mm. or less in diameter like that figured on PL XVII., Fig. 7. 



Forraation of Corpora lutea. — My observations on the for- 

 mation of the corpora lutea differ somewhat from those of 

 Mr Poulton for Ornithorhynchus. The fibrous layer of the 

 follicle becomes very thick, and the follicle itself becomes 

 filled with a homogeneous-looking mass (PL XVIL, Fig. lb), 

 which stains readily with Borax carmine; trabeculm of the 

 fibrous tissue outside the follicle grow into it, and gradually 

 obliterate the cavity ; the mass in the centre of the follicle 

 does not disappear for a considerable time, but remains 

 divided into a number of fragments by the trabecular out- 

 growth of the follicular wall. The peculiar yellow cells (a) 

 appear in the interior of the trabecidce, and increase in 

 number, finally occupying the whole of the follicle. The 

 appearance of the yellow cells which constitute the cor^ms 

 liUeum at its last stage of degeneration is extremely like 

 that of the cells which are supposed to be the remains of the 

 Wolfiian tubules. 



The results contained in this paper show that in every 

 essential point in the structure of the ovum and its follicle, 

 Fchidna agrees with Ornithorhynchus, as described by Mr 

 Poulton. They may be briefly stated as follows : 



(1.) The follicular epithelium continues to invest the ovum 

 throughout its sojourn in the Graaffian follicle as a 

 single layer of cells only,^ as in the lower Yertebrata. 



1 I am not quite certain as to this point. In the large ovum figured on 

 Plate XVIL, Fig. 7, I have indicated two rows of follicular cells, and occa- 

 sionally there appeared to be three. Two or three layers of follicular cells 

 are found in the ovarian ova of birds, and I am therefore inclined to think my 

 observations are correct and not based upon defective preparations. 



