THE CEPHALIC NAVEL. 



239 



The membrane soon separates from the embryo, and later from the blasto- 

 derm. The blastoderm cells meantime become very deep and columnar, the 

 nuclei and dense cytoplasm collecting at their outer, and numerous yolk par- 

 ticles, at their inner ends. (Fig. 129, d.o.) At this period, the blastoderm cells 

 beyond the embryonic area are everywhere sharply cut off from the yolk, and there 

 is no migration of cells from one to the other. 



In stage G (Fig. 143), the blastodermic nuclei, along a narrow zone just 

 beyond the germ wall, take on a sharper and darker color, do. During stages 



FIG. 142. Same, stage K, mercator projection. On the right, the ends of the thoracic appendages are 



removed. 



H and 7, the zone widens, gradually spreading over the entire blastoderm. This 

 change in the appearance of the nuclei marks the beginning of a rapid prolifera- 

 tion, and subsequent degeneration of the blastoderm cells. During this process, 

 the chromatin collects into larger, intensely stained particles; the columnar cells 

 divide, take on a spherical, or oval form, and pass in great numbers into the yolk, 

 where they form a very conspicuous mass of loosely arranged cells. (Fig. 134, d.o.) 

 The cytoplasm of these cells soon becomes fainter and more transparent, and 



