PROGRESS FROM THE GENERAL TO THE SPECIAL. 



025 



spheres, each of which contains a number of spherules similar to the smaller 

 spherules; it presents also some vacuoles in the region of the cicatricula. 

 At the point where the white yolk dips into the yellow yolk is a depression, 

 which is occupied by a small white disk, about the sixth of an inch in di- 

 ameter, termed the blastoderm or cicatricula (bl). The disk is always upper- 

 most when the egg is opened from the side, in consequence of the lighter 

 specific gravity of this part of the vitellus. A typical cicatricula presents 



Fir,. 324. 



Diagrammatic Section of an Unincubated Fowl's Egg: bl, blastoderm ; wy, white yolk. This con- 

 sists of a central flask-shaped mass and a number of layers concentrically arranged around this; yy, 

 yellow yolk; vt, vitelline membrane; x, layer of more fluid albumen immediately surrounding the 

 yolk ; w, albumen, consisting of alternate denser and more fluid layers; cfil, chalaza; ach, air-chamber 

 at the broad end of the egg. This chamber is merely a space left between the two layers of the shell- 

 mernbraue. ism, internal layer of shell-membrane ; sm, external layer of shell-membrane; s, shell. 



an outer white rim, or area opaca, a circular transparent area, or area pel- 

 lucida, and a more or less well-marked central opacity. The area opaca and 

 central opacity appear to be due to the disposition of the white yolk beneath 

 the blastoderm, whilst in the part corresponding to the area pellucida, the 

 blastoderm is separated from the white yolk by a space, filled with a little 

 fluid. Imbedded in the protoplasm of the germinal disk is a globular or 

 ellipsoidal highly refractile body, termed the germinal vesicle; in the interior 

 of which, again, is a small body, the germinal spot. 



776. Before being laid, and at about the time when the shell is being 

 formed around the egg, the cicatricula or germinal disk undergoes a remark- 

 able process, termed segmentation. This commences with the formation of a 

 furrow, which is quickly crossed by another at right angles, and soon by 

 others (Fig. 325). The examination of hardened specimens shows that the 

 segmentation is not confined to the surface, but extends through the mass of 

 the blastoderm, and that it takes place not only by vertical but by horizon- 

 tal furrows, that is, by furrows parallel to the surface of the disk. The 

 original germinal disk is thus broken up into a number of small masses of 

 protoplasm, which are smallest in the centre of the disk, and upon the sur- 

 face ; the smaller size of the superficial layer foreshadowing the subsequent 

 division of the blastoderm into two layers. The disk having undergone 

 segmentation becomes separated from the white yolk, on which it has hitherto 

 been lying, by a space containing fluid, termed the segmentation cavity (see 



