432 
DR. W. H. RANSOM ON THE OVUM OF OSSEOUS FISHES. 
This substance has an alkaline reaction on red litmus-paper, but does not change 
turmeric. A very weak solution of potash destroys its viscidity, and permits water to 
enter and distend the egg. A weak solution of acetic acid destroys its viscidity, and 
renders it opaque and flocculent. Boiling and nitric acid do not coagulate it. 
b. The yelk-sac is a rather thick membrane, measuring It surrounds the yelk- 
ball, and is, on its inner surface, in immediate contact with it, so long as no water has 
been imbibed. It is characterized by a tolerably regular tine dotting, the dots being 
arranged in lines which cross each other, so that lozenge-shaped spaces are left between 
them. At the folded margin corresponding radial lines appear, themselves resolvable 
into dots, as if the membrane consisted of concentrically arranged laminae, each dotted 
so as to correspond with the next layer. The further details of this structure will be 
given in the section devoted to the development of the ovarian ova. 
One part of the surface of these eggs is distinguished from the rest by a scattered 
group of stalked, cup-shaped processes, or buttons, which covers about one-fourth of the 
surface, and marks the germinal pole of the unimpregnated egg. The form of these 
buttons varies a great deal, and the size is by no means constant, but for the most part 
they a little exceed in length the thickness of the yelk-sac, and the form, when not 
changed mechanically, is shown in Plate XV. fig. 2. Further details of their structure 
are given in the section on development of ovarian ova. 
In the centre of this group of buttons is the micropyle. It may be seen in various 
modes. I first noticed it while crushing an unimpregnated egg in June 1854, but its 
characters are best studied by removing the germinal segment, and examining it sepa- 
rately after washing away the contents. Sections may also be made by imbedding ova in 
strong warm jelly, and slicing the mass when cold, and in this way, some may be ob- 
tained which cut the micropyle vertically. It consists of a wide-mouthed, funnel-shaped 
pit, directed towards the centre of the egg, near its apex becoming more acute, and 
terminating in a short narrow tube with almost parallel sides (Plate XV. fig. 3). The 
inner end of this tube is apparently open, and however viewed, whether from within or 
without, whether in sections or in whole eggs, looks like a clear, pale-blue, oval or cir- 
cular aperture, and measures YWto" (Plate XV. fig. 4). The fine dottings of the yelk-sac 
cease abruptly at the margins of this opening. Powers of xlOO to X 200 are well 
suited for the examination of these sections, but higher ones may be used without diffi- 
culty. To examine the relation of the micropyle to the egg as a whole, or to the yelk- 
ball, powers of X 50 to xlOO are more convenient. 
Unimpregnated ova yield to moderate pressure without rupture, and when the ger- 
minal pole presents, the micropyle may be seen either full face, or at various inclinations ; 
and if the position be suitable, the terminal opening is still visible as a clear bluish spot, 
although the whole of the yelk-ball is below it. When the germinal pole is in profile 
under suitable pressure, the funnel is seen projecting into that portion of the yelk-ball 
which corresponds to this pole (Plate XV. fig. 5), and which I have called the discus 
proligerus. In unimpregnated ova this relation is not quite so distinct as in those which 
