70 



OVUM. 



The cicatricula of the unfecundated egg, 

 such as is laid by fowls secluded from the 

 cock, differs from that now described princi- 

 pally in the absence of the marked dis- 

 tinction between the central clear and the 

 peripheral opaque part. The germinal ve- 

 sicle, which to all appearance remains the 

 same in the ovary till the yolk leaves the 

 ovarian capsule, is now no longer to be seen; 

 and the cicatricula is often marked irregularly 

 throughout, but more especially towards the 

 circumference, with clearer intervals, or small 

 irregular circular orjoval spaces, mingled with 

 the opaque substance of the disc. I have, 

 not, however, had the means in more recent 

 times of making a sufficiently careful exami- 

 nation of the cicatricula in this condition to 

 enable me to state more minutely in what 

 respects it differs from that of the fecundated 



egg- 

 In the ovarian yolk, while still within its 



capsule, a white spot corresponding to the 

 cicatricula also exists, and occupies the same 

 place in relation to the yolk cavity and canal. 

 Its structure and appearance, however, are 

 somewhat different from that of the true cica- 



Fig. 51. 



Cicatricula, and its contents, in the ovarian egg of 

 the Fowl. 



A. A square portion of the surface of the ripe 

 ovarian yolk, showing the vitelline disc or cicatricula, 

 with the germinal vesicle in the centre, magnified 

 ahout six diameters. 



u. Lateral view of the same, to show the con- 

 vexity produced by the thickness of the disc round 

 the germinal vesicle. 



c. Vertical diagrammatic section of the same ; m, 

 vitelline membrane; d, granular disc ; g, germinal 

 vesicle. 



D, E, F. Germinal vesicles more highly magnified; 

 D, from a yolk of about one tenth of an inch dia- 

 meter, showing scattered globules or germinal 

 spots; E, from a nearly ripe 3 - olk, quite clear; F, 

 from another of the same period, exhibiting a turbid 

 or minutely granular mass from the action of water. 



tricula of the egg which has passed through 

 the oviduct ; it is covered by a layer of closely 

 set nucleated cells which lie below the vitel- 

 line membrane ; it contains the germinal vesicle 

 in its centre, and, instead of being thinnest 

 towards the middle, the mass of its granular 

 substance is accumulated in greater quantity 

 in that part round and below the germinal 

 vesicle, and thins gradually off towards the 



margin. Nevertheless, its much lighter colour 

 than the surrounding part of the yolk makes 

 it always easy to distinguish it. Its margin, 

 however, is not so well marked as that of the 

 true cicatricula ; for the opaque whitish sub- 

 stance seems there gradually to pass into or 

 be continuous with the most superficial layer 

 of cells covering the yolk. To this ovarian 

 representative of the cicatricula, Von Baer 

 has given the name of stratum proligcmm. It 

 is also somewhat smaller than that of the laid 

 egg. It is usually to be found on that part 

 of the yolk which is next the ovary, which, 

 as the yolk hangs within its capsule in the 

 usual attitude of the bird, will be upper- 

 most, and for the most part is situated close 

 to the pedicle of the ovarian capsule. This 

 position is not, however, a constant one ; for 

 sometimes the cicatricula is seen on the sides 

 of the yolk, or towards the stigmatic band 

 of the capsule, but rarely, it would appear, 

 towards the ends or poles of the yolk. 



The cicatricula may generally be perceived 

 on the surface of the yolk when the outer- 

 most layers of the capsule have been re- 

 moved, and the germinal vesicle can be distin- 

 guished in it shining through the inner layer of 

 the capsule and the vitelline membrane. It is 

 placed close below the nucleated cells which 

 line the latter, and adheres along with them 

 somewhat to its inner surface; so that in gene- 

 ral, it is easiest to remove this disc along with 

 a portion of the vitelline membrane, when it is 

 desired to obtain it for separate and more mi- 

 nute observation by transmitted light. The 

 vitelline membrane being cut round with scis- 

 sors at a short distance from the margin of 

 the disc, the parts are floated off in water or 

 serum, and then may readily be separated with 

 a little careful manipulation. 



The germinal vesicle, or vesicle of Pur- 

 kinje, may always be seen with the unassisted 

 eye, with a good light, in the centre of the 

 ovarian cicatricula, or proligerous disc, in all 

 ripe ovula, and in most of those which are 

 above a tenth of an inch in diameter. It 

 constitutes there a well-defined shaded cir- 

 cular spot, from ^ to -^ of an inch in diameter. 

 When the proligerous disc alone has been 

 removed Tor observation and laid on a flat 

 surface, and viewed somewhat from the side, 

 or when the granules are torn asunder with 

 needles, so as to make a partial section 

 of it without removing or bursting the ger- 

 minal vesicle, it is easy to perceive that the 

 middle part, containing the vesicle, is more 

 elevated than the rest; and that, although 

 the substance of the disc seems to pass quite 

 smoothly or evenly from the sides over the 

 germinal vesicle, the granules of the disc en- 

 velope the vesicle only slightly, and none 

 cover its middle part: the vesicle, there- 

 fore, is set, as it were, in a depression of the 

 disc, which fits round and overlaps its 

 margins, and a considerable thickness of gra- 

 nular substance is continued in the disc below 

 the vesicle. (See/g. 5!, in section). 



If we select for examination the most ad- 

 vanced yolk of the ovary, which, in a hen laying 

 daily, or almost daily, would probably have 



