730 FORMATION OF THE EMBRYO. 



birds, within the ovarian follicle, and is, in respect to its volume, the 

 main constituent of the ovarian egg. 



At one point upon the surface of the yolk of the fowl's egg, while still 

 contained within the ovarian follicle, there is a whitish circular spot 

 about 5 millimetres in diameter, lying immediately beneath the vitelline 

 membrane. This is the cicatricula. It is a thin layer of uniformly 

 granular material, containing none of the spherical bodies found in the 

 white and yellow yolk. Its granules are imbedded in a homogeneous 

 substance of viscid consistency, by which they are agglutinated into a 

 disk-like mass. In its centre is contained the germinative vesicle, 

 which is distinctly visible by its transparency and well-defined outline, 

 until the mature egg is ready to leave the ovary, when it disappears, 

 as in other classes of animals. The cicatricula of the fowl's egg cor- 

 responds, therefore, in its structure, though not in its form, with the 

 entire vitellus of the mammalian egg. Its position is always exactly 

 above the tubular prolongation of white yolk, already described as 

 leading to the central cavity of the egg. 



Formation of the Blastoderm. The fowl's egg is fecundated soon 

 after leaving the ovary, and while in the upper portion of the ovi- 

 duct. The segmentation of the cicatricula then begins, by a furrow 

 which passes across its disk, and which is followed by others running 

 in different directions. By the continued multiplication of these fur- 

 rows, the substance of the cicatricula is divided successively into 

 smaller and smaller portions; the process beginning and proceeding 

 most rapidly at its centre, but extending thence outward to the peri- 

 phery. When these divisions have become reduced in size and increased 

 in number to a certain degree, they present, as in other instances, the 

 form and structure of distinct cells. The cells are in two layers. Those 

 of the upper layer are smaller, more numerous, cylindrical or prismatic 



Fig. 250. 



VERTICAL SECTION THROUGH A PORTION OF THE BLASTODERM of a fowl's 

 egg, at the commencement of incubation. 1. Upper cellular layer. 2. Lower cellular layer. 

 3, 3. Larger cells, found in small number beneath those of the lower layer. (Foster and Bal- 

 four.) 



in form, standing upright side by side, like the cells of columnar epithe- 

 lium, and adherent to each other by their adjacent surfaces. According 

 to Foster and Balfour 1 they have a very uniform size of 9 mmm., and 

 most, if not all of them are provided with a distinct oval nucleus- The 



1 Elements of Embryology. London, 1874, p. 17. 



