THE OVUM IN THE AMPHIBIA. 
185 
that this appearance is in reality a vesicle, and therefore am content to describe it as 
a spot, although it conveys the idea of being a vesicle. In some ova there are two, 
four or six of these spots imbedded each in a small portion of white substance. When 
only a single spot exists, the white surface of the egg for some space around it is more 
defined than afterwards, and exhibits faint indications of a crucial division of the 
yelk on this surface immediately around the spot. This is the condition of a few ova 
immediately after spawning; but the majority have then advanced farther in their 
changes, and show four or six rounded dark-coloured spots at a little distance in the 
place of the single central one. When four spots occur they are usually arranged in 
a quadrangle, and are less than their own diameter apart. They convey the idea 
of being derived from the central one ; but I have never seen any division of this, 
and if such division takes place, I think it must occur before or at the very moment 
the ova are expelled. In a further advanced stage of the ovum the four dark spots 
have become larger, and are each imbedded in a distinct portion of the white surface. 
One minute after deposition the spots are more widely separated, and are then each 
encircled by a separate patch of white substance. Two minutes after spawning six 
dark spots have made their appearance, one of which is situated nearly in the centre, 
and the remaining five are so arranged around this that the white patches in which 
they were imbedded seem to have coalesced. In three minutes the spots are further- 
enlarged, and appear joined by a dark line of colour extending fr-om each, so that the 
whole form, as it were, a knotted ring that includes a patch of the white surface of 
the yelk with one of the dark spots near the centre. In four minutes the ring 
around the included white substance is more distinct, and the white surface of the 
egg has increased in extent. In a further advanced stage at this period the white 
portion included in the ring exhibits the appearance of a white, very opake patch, the 
dark spot in the centre having disappeared. Around this opake white patch is the 
dark-coloured knotted ring, now become more uniform, and resembling a ragged 
chink or slight circular furrow or division in the white surface. The centre of this 
hemisphere of the egg thus comes to be occupied by a white patch instead of the 
dark spot. At Jive minutes this central white patch, — which, as before stated, and 
from what afterwards occurs, may readily be mistaken, on casual inspection, for the 
germinal vesicle, altered in its appearance and arrived at the surface, — were it not 
that we now know that this has long before entirely disappeared, — becomes more 
defined, and the dark circle around it is more uniform and distinct. At ten minutes 
the central patch is a little reduced in size, and the circle that incloses it begins to 
take the appearance of a diffused halo. At Jifteen minutes the central white patch is 
more reduced, and the halo is spread wider, while the whole of this hemisphere of the 
egg has acquired a whiter appearance, and become more distinct from the dark 
colour of the sides and future dorsal hemisphere. At twenty minutes the central 
patch has become still smaller and rounder, and the dark halo much broader. 
At this period an interesting circumstance occurs which may hereafter be found 
MDCCCLI. 2 B 
