272 HOLMES. [Vol. XVI. 



Eggs treated by the foregoing method will keep indefinitely, 

 neither fading nor becoming opaque. When the ^gg is mounted, 

 the stain is usually safe. In successful preparations the cells 

 of the tgg are not strongly darkened, but the cell boundaries 

 stand out in a remarkably sharp and clear manner. Some cells, 

 however, stain much darker than others. The trochoblasts and 

 the cells of the head vesicle remain almost perfectly transparent, 

 while the cells of the cross are colored brown. Owing to the 

 transparency of the trochoblasts the cross appears with a won- 

 derful distinctness, enabling one to orient the Q:gg at a glance. 

 Nuclei are not stained, but they can usually be seen, and the 

 spindles of dividing cells are often visible. The eggs may, 

 however, be stained so that the nuclei show fairly well, but I 

 have generally dispensed with nuclear staining when using this 

 method. The treatment often injures the impregnation and 

 renders the Q.gg more opaque, so that it is more often a nui- 

 sance than a benefit. Individual differences in the impregna- 

 tion of different eggs are quite decided. Even among eggs 

 from the same capsule subjected to exactly the same treat- 

 ment, some will be strongly stained, while the staining in 

 others is faint. If eggs are left too long in the nitrate, they 

 not only become too dark, but also become brittle, and the cells 

 break or become separated when the eggs are rolled. Cell 

 boundaries take the stain at all periods in the development of 

 the Qgg, even in the very first cleavage stages. The method 

 is of special value, however, in following the cell lineage of 

 organs in the later periods of cleavage. I have obtained won- 

 derfully clear preparations of gastrulae, which show the bound- 

 aries of each cell, when there are several hundred cells in the 

 ^%,g, with diagrammatic distinctness. The cells of the proto- 

 troch in the gastrula stage form a conspicuous transparent 

 band, which appears in marked contrast to the adjacent cells 

 — a circumstance which proved very helpful in tracing out the 

 cell lineage of this organ. 



