Mr F. E. Beddard on the Ovary of Echidna. 359 



a distinct wall from the surrounding substance of the ovum ; 

 as a rule it appears to be eccentric in position even in the 

 youngest ova, though occasionally it is nearer to the centre. 

 The contents of the germinal vesicle present the appearance 

 of being fluid or semifluid ; in most cases its substance had 

 shrunk away from the wall, and filled only half the space. 



In ova about twice the size of those just described, and 

 figured in PL XVII., "Fig. 6, the vitelline membrane appears 

 as an obvious limiting membrane of the ovum. In a more 

 mature ovum, such as that figured on PI. XVII., Fig. 7, the 

 following structures can be easily recognised : The ovum is 

 bounded externally by a stoutish homogeneous membrane {m), 

 which stains deeply with Borax carmine ; this appears to be 

 the zona pellucida. It is about half the diameter of the 

 follicular epithelium (/), from which it is naturally sharply 

 marked off as from the subjacent yolk. Generally speaking, 

 however, the ovum, when it shrinks away from the wall of 

 its follicle, carries the follicular epithelium with it. The 

 zona pellucida appears to me to be the only egg membrane 

 in Echidna. I did not succeed in recognising any of the 

 subjacent layers figured and described by Mr Poulton in 

 Ornithorhynchiis ; only the central portion of the membrane 

 presented the appearance of a greater density than the outer 

 and inner layers, and in the sections studied by me was 

 somewhat more deeply tinged by the staining re-agent. I 

 am not at all clear, however, that this was not an optical 

 delusion caused by the displacement of the follicular cells, 

 or the crumpling of the zona pellucida itself, inasmuch as I 

 was unable to trace the darker band round the whole circum- 

 ference of the ovum, and I only occasionally noticed its 

 presence. Immediately beneath the zona pelhccida is a layer 

 of finely granular yolk, the extent of which is about equal to 

 the diameter of the follicular epithelium and the zona pel- 

 hccida taken together ; this peripheral layer of the yolk is 

 hardly at all affected by Borax carmine. The main portion 

 of the ovum is made up of a protoplasmic reticulum, indi- 

 cated by the blacker lines on Fig. 7, which becomes very 

 deeply stained by Borax carmine, and in successful prepara- 

 tions is for this reason extremely obvious. The meshes of 



