106 Prof. Owen on the Echidna Hystrix. [Mar. 2, 



greater than, equal to, or less than (I) (Art. 9, iii.). Lastly, every circle 

 through F whose centre is on AF is inverse to a circle of the same kind as 

 itself, as is also every circle passing through the points A and F. 



(5) When the origin A is on the fundamental conic (F) the fundamental 

 points B and C coincide with it, and the three principal lines coincide with 

 the tangent at A. The conic (I) inverse to the line at infinity touches (F) 

 at A and bisects all its chords through A, and the conic inverse to every 

 other line in the plane has obviously threte-pointic contact at A with (F). 

 All lines which converge to one and the same point on the tangent at A 

 are inverse to conies which have, at A, four-pointic contact with each other ; 

 hence right lines parallel to the tangent at A are inverse to conies having, 

 at A, four-pointic contact with (I). Finally, the tangents to any two 

 inverse curves, at inverse points p, p', intersect on a line D which touches 

 (F) at the intersection of this conic and pp'. 



A still more special case, into the details of which we cannot enter, is 

 when the fundamental curve is a parabola, and the origin at the infinitely 

 distant extremity of its axis. Inversion relative to a conic and its focus is 

 also a special case which merits attention, but cannot be here considered. 



Transformation correlative to quadric inversion. 



16. Corresponding to the method of quadric inversion, there is of 

 course a correlative method, which in certain inquiries is equally useful. It 

 does not, however, require a separate exposition. It may also be remarked 

 that the reciprocal polar, relative to the fundamental conic, of the inverse 

 of any primitive curve, and the inverse of its reciprocal polar, lead at 

 once to derivative curves, of which negative and positive Pedal Curves 

 are special cases. 



II. " On the Marsupial Pouches, Mammary Glands, and Mammary 

 Foetus of the Echidna Hystrix." By Professor OWEN, F.ll.S. 

 Received February 18, 1865. 



(Abstract.) 



In a former communication on the generative economy of the Monotrc- 

 mata*, the author showed that the ovum left the ovarium with a spherical 

 vitellus 1-i line in diameter, and had attained a diameter of 3 lines in the 

 uterus, the increase of size being due to increase of fluid between the 

 chorion and vitelline tunics. This fluid, homologous with the albumen of the 

 egg of oviparous vertebrates, did not coagulate in alcohol, and the only change 

 presented by the vitellus of the largest observed ovum was a separation 

 from the " food-yolk" of a " germ-yolk " in the form of a stratum of very 

 minute granules, adhering to part of the membrana vitelli. There was no 



* " On the Ova of the Ornithorhynclms paradoxus" Philosophical Transactions, 

 vol. cxxiv. p. 555 



