OVUM. 



69 



towards the cicatricula, near which it again 

 widens and spreads out like a shallow cone. 

 This whiter internal substance constitutes 

 what has been called the central cavity (or 

 latebra) of the yolk : the whole of this inner 

 part has something of the shape of a flask, 

 with a narrowing neck and a wider mouth 

 ut the top, which is, as it were, surmounted 

 or closed in by the cicatricula. (See^g. 49.) 



The shape of the yolk, I have said, is not 

 that of a regular ellipsoid ; the less density 

 of the upper part, which is towards the cica- 

 tricula, giving rise to a widening of the yolk 

 on that side, as may be seen in fig. 44, A, which 

 represents a vertical section of an egg boiled 

 while lying on its side. This does not depend 

 simply on the rising of oil globules in greater 

 quantity to the upper side of the yolk, but, 

 as has already been noticed, on the fixed 

 predominance of globules containing oil in 

 the neighbourhood of the cicatricula. 



Neither is the outer deeper-coloured por- 

 tion of the yolk altogether uniform in structure 

 or appearance ; for it will be seen, both in the 

 raw and boiled egg, but most easily in the 

 latter, that several concentric layers surround 

 the central cavity and canal of the yolk, as 

 well as the funnel-shaped dilatation which 

 lies below the cicatricula. These layers are 

 marked by a slight variation in colour, and 

 are attended by a difference in the minute 

 structure of the corpuscles composing the 

 alternate layers. They probably depend upon 

 the growth of the coloured part of the yolk 

 being more or less rapid at different successive 

 periods. 



The cicatricula of the newly laid egg is a 

 spot of an opaque yellowish white, easily dis- 

 tinguished by its difference of colour from the 

 rest of the yolk, about one sixth of an inch 

 in diameter, and King immediately within the 

 vitelline membrane, in connection at its mar- 

 gins with the most superficial layer of the 

 yolk substance. Examined in a favourable 

 light* it will be found, that in the laid egg, 

 when fecundated, the cicatricula consists "of 

 a central clearer and thinner part, and of an 

 external more opaque annular portion. The 

 central part is about one third the diameter 

 of the whole, and seems as if it perforated 

 the remainder of the disc with a circular 

 aperture, something after the manner of the 

 pupil of the iris. There is not, however, any 

 perforation in reality, but only a greater thin- 

 ness and transparency of the central part of 

 the disc. Neither is this central part entirely 

 clear; for there is placed below its middle a 

 round heap of whitish granules, described 

 by Pander as the nucleus cicatriculce (see the 

 figure in section), which gives greater opacity 

 to that part when viewed directly from above. 

 The central part of the cicatricula, already 



* It may be here mentioned, that by far the best 

 mode of examining the natural appearances of the 

 parts as they lie in the opened egg, is to allow a 

 ray of strong or of direct sunlight to fall upon the 

 pa'rt which it is wished to investigate, through an 

 aperture in a screen, which places the rest of the 

 egg and the observer in comparative darkness. 



obvious when the egg is first laid, is the same 

 which, after some hours of incubation, ex- 

 pands, changes its figure, and becoming still 



Fig. 50. 



Structure of the cicatricula in a laid FowPs egg. 



A. Diagrammatic section of the yolk near the 

 cicatricula, enlarged; a, vitelline membrane; b, 

 cicatricula; c, nucleus; d, canal leading to the 

 cavity ; e, e, large yolk corpuscles of the coloured 

 part: the corpuscles are not represented of their 

 real proportional sizes, but more with a view to 

 show their general difference. 



B. Enlarged view of the cicatricula, as seen from 

 above on the surface of the yolk in an impregnated 

 egg : the dark central space or transparent area 

 surrounded by an opaque zone and one or two 

 delicate haloes. 



c. Cicatricula of an unfecundated laid egg : instead 

 of the central transparent area a number of rather 

 irregular transparent spots are seen. 



more clear, receives the name of transparent 

 area, in the centre of which the embryo be- 

 gins to be formed; while the outer more opaque 

 part retains its greater thickness, and is con- 

 verted afterwards into the vascular and peri- 

 pheral part of the germinal membrane which 

 spreads over the yolk. Round the margin of 

 the cicatricula the deeper-coloured yolk sub- 

 stance is seen even in a perfectly fresh or 

 newly laid egg to be intersected by one or more 

 fine circles of a lighter colour. These seem 

 to be the same which afterwards, expanding 

 and widening, constitute the haloes which pre- 

 cede and accompany the extension of the ger- 

 minal membrane over the yolk. These circles 

 are the terminations at the surface of the 

 concentric layers of lighter substance, which, 

 as already mentioned, may be seen surround- 

 ing the central cavity and canal of the yolk 

 (see fig. 49). It seems not improbable that, 

 the difference in the structure of the central 

 and peripheral parts of the cicatricula just 

 stated proceeds from, or is connected with, 

 the peculiar process of fissuring or segmen- 

 tation which follows the disappearance of the 

 germinal vesicle from its central part in the 

 fecundated egg ; but the description of this 

 process belongs to a later section of the, 

 present chapter. 



F 3 



