FORMATION OF THE EMBRYO IN THE FOWI/S EGG. 625 



segments by transverse and oblique furrows of communication (Fig. 

 182, III.). By the continuance and peripheral extension of this process 

 the area of segmentation is gradually divided into small polygonal 

 bodies (Fig. 182, IT.), many and finally all of which are provided with 

 a central nucleus, and which are accordingly regarded as nucleated cells. 

 The study of perpendicular sections of the cicatricula in this condi- 



FIG. 182. 



PHASES OF SEGMENTATION IN THE CICATRICULA OF THE FOWL'S EGG, within the oviduct. 



(Kolliker.) 



tion shows that its segmentation extends not only in a lateral direction, 

 but also throughout its depth. The furrows which appear to divide it 

 into isolated parts are at first only superficial, and its surface is already 

 subdivided while its deeper portions are still entire. But the process 

 of division continues from above downward until it occupies the whole 

 thickness of the plastic vitellus, to the surface of the white yolk. By 

 this means the cicatricula is converted into a disk-like mass of nucleated 

 cells, and is then known as the " blastoderm." 



2P 



