22 J. P. HILL. 



plasm, close below and at right angles to the zona. Its exact 

 site is subject to some slight variation, and is best described 

 as adjacent to the equatorial region of the egg, sometimes 

 nearer the lower pole, more usually, perhaps, nearer the 

 upper. Centrosomes and polar radiations were not observed. 

 The heterotypical chromosomes (geraini) have the form ot 

 somewhat irregular, more or less angular granules. I have 

 not been able to determine their number. The figure is 

 barrel-shaped, and almost as broad as long, measuring 

 •015 X -013 mm. The first polar body (fig. 6, 'p.b'^.) is small 

 relatively to the size of the egg, its diameters varying round 

 •03 X '01 mm., and its shape is that of a flattened bi-convex 

 disc. In uterine eggs there is some evidence pointing to the 

 probability of its having undergone division. 



The second polar spindle (figs. 3 and 7) lies immediately 

 subjacent to the first polar body in the fully ripe ovarian 

 ovum. It is shorter than the first, measuring 'Olo mm., and 

 much narrower. The second polar body measures about 

 ■015 X *01 mm. in diameter, and is thus smaller than the first. 

 I have only seen the second polar body in uterine ova, and 

 therefore can only presume that it is separated ofl^ in the 

 upper part of the Fallopian tube, subsequently to the pene- 

 tration of tlie sperm, as in Eutheria. 



Ovulation takes place irrespective of whether copulation has 

 occurred or not, and it is a fact worthy of record that, even 

 if the ova be not fertilised, the pouch and mammary glands 

 undergo the same series of growth changes as are charac- 

 teristic of, at all events, the eai-lier stages of normal 

 pregnancy. 



The follicular cells of the discus proligerus investing the 

 ovum are already in the ripe follicle in a state of disruption, 

 and I believe they separate completely from the ovum at the 

 moment of dehiscence, so that, except for the zona, the ova 

 are quite naked when they enter the tube. I have no evidence 

 of the existence outside the zona of a layer of proalbumen 

 such as Caldwell describes round the ovum of Phascolarctus, 

 Apparently the ova are shed almost simultaneously, and they 



