87 



pear altogether, or are transformed into others of a different colour ; 

 and, lastly, the germinal vesicle is destroyed. 



By tracing the changes of the ovule in unfecundated females of 

 the same species, the author finds the disappearance of the vitelline 

 granules to be dependent upon, while the formation of the chorion 

 is wholly independent of, the influence exerted by the spermatic 

 particles on the ovule. 



As soon as the vitelline granules and germinal vesicle have dis- 

 appeared, the whole interior of the chorion is filled with a clear fluid, 

 in which a few granules and the germinal spot are seen to remain. 

 By swelling up this constitutes the embryonic vesicle and spot. A 

 membrane separates from the interior of the chorion, and contracting 

 on the granules forms a spherical yolk, in the centre of which is the 

 embryonic vesicle. This is the perfect ovum. The subsequent di- 

 visions of the embryonic spot, vesicle and yolk are described ; the 

 author particularly pointing oiU the gyrations of the embryonic ve- 

 sicle immediately after di\'ision. As soon as the whole interior of 

 the egg has been filled by the subdivisions of the yolk, the external 

 granules coalesce and form a continuous membrane internal to the 

 chorion, which by gradual depression on one of its sides forms first 

 a fleshy cup, and then, as the membrane of its concavity touches that 

 of its convex surface, acquires the form of a ring. The ring divides 

 at some point of its circumference, the extremities become pointed, 

 and thus the young Ascaris receives its characteristic shape. The 

 author has frequently repeated his observations with a view to their 

 verification, and has employed the camera lucida to render the illus- 

 trative figures as accurate as possible. 



9. " On Induced and other Magnetic Forces." By Sir W. Snow 

 Harris, F.R.S. &c. Received April 29, 1851. 



The question as to identity in the source of those several and 

 mysterious powers of nature by w^hich masses or particles are moved 

 either toward, or from each other, being a question of deep physical 

 interest, the author of this paper has been led to some further in- 

 vestigation of the nature and laws of magnetic force, in the course 

 of which several new facts have presented themselves which he 

 thinks not altogether unworthy of attention. 



Magnetic attraction as commonly observed being found to depend 

 on certain impressions made on the attracting bodies usually de- 

 signated by the general term induction, it appears essential to the 

 progress of any inquiry into the laws of those forces operating ex- 

 ternally to a magnet through space, to commence with a rigid exa- 

 mination of the nature and mode of action of those inductive forces 

 upon which the reciprocal force of attraction between the bodies 

 immediately depends. These forces of induction may be considered 

 as a series of successive or reverberating influences, operating be- 

 tween the near and opposed surfaces of the magnetic bodies. When, 

 for example, a magnet is opposed to a mass of soft iron, a direct 

 impression is first made on the iron by which the iron is rendered 

 temporarily magnetic ; this induced force operates in its turn by a 



