No, -2.] THE OOGENESIS OF BUFO LENTIGINOSUS. 371 
Of the great variety of combination stains that have been 
experimented with at different times in the hope that it might 
be possible to differentiate the chromatin from the nucleoli, 
safranin and gentian violet, used in the manner recommended 
by Hermann (39), proved to be by far the best. This stain is 
rather difficult to use, but when a satisfactory preparation has 
been made, all of the plasmosomes are stained a vivid red and 
stand out in sharp contrast to the chromatin which is a deep 
blue, while the structures which I have called “compound- 
nucleoli” are stained purple. Much of the material was stained 
with iron hematoxylin and orange G. This combination does 
not differentiate the nucleoli from the chromatin; but it gives 
such clear, sharp outlines that it is of great value in studying 
the early stages in the development of the oocytes when the 
chromatin stains but faintly and cell boundaries are difficult 
to determine. Borax carmine combined with Lyon’s blue, 
safranin followed by Lichtgriin, and Delafield’s hematoxylin 
with orange G. also give good preparations, particularly of the 
later growth stages of the oocytes. 
Il. THe PRIMORDIAL GERM-CELLS. 
Owing to the large amount of yolk in the embryo and to 
the vagueness with which the cell boundaries are defined, it is 
impossible to trace the germ-cells in Bufo back to the segmen- 
tation stages of the egg, as has been done in other more favor- 
able forms. Not until a tadpole is five or six days old and has 
attained a length of about 4 mm. can one point out definitely 
the group of cells that will develop into the genital ridge. 
At this stage of development the lateral plates of mesoderm 
(Fig. 1, L. M.) are well defined; the cells are small, with 
clear outlines, and they contain but comparatively little yolk. 
In the endoderm (Fig. 1, E.) on the contrary, the cells are 
very large; they are filled with yolk spherules which stain very 
deeply with iron hematoxylin, and their boundaries are ir- 
regular and difficult to determine. In the mid-dorsal region 
of the embryo there is usually found at this time a ridge of 
