44 J. B. Johnston. 
of the mesenchyme retain large quantities of yolk, but these do not 
enter in any way into the structures to be studied in this connection. 
The ectoderm and entoderm are drawn as faithfully as possible in 
the figures. When it is remembered that the general gray shading 
of the cells represents fuchsin stain, while only the granules and 
chromatin are black, it will be seen that the distinction between ecto- 
derm and entoderm is many times more striking in the preparations 
than it is in the figures. In late stages, when the yolk begins to 
be used up in the entoderm, the sharp staining of the granules is of 
the greatest advantage. Before that time the yolk has entirely dis- 
appeared from the ectoderm and most of it from the nervous system. 
In such a study as this good serial sections without tearing or 
distortion are necessary, and the yolk itself is well known as a great 
obstacle to obtaining such sections. I have tried some methods of 
fixation in which the yolk is said to be made soft and easily cut. I 
have not found, however, that such embryos show such perfect fixa- 
tion as I desired for the study of the early stages in the formation 
of taste buds. The fixing fluid used renders the yolk hard and brittle, 
but fixes faithfully all elements of the tissues and without any dis- 
tortion or shrinking. To obtain perfect serial sections in the three 
planes I used the method of imbedding in a paraftin-rubber mixture 
made as follows: to 100 ce. of paraffin which melts at 1 or 2 degrees 
higher temperature than desired in the imbedding mass, add 1 to 
9 grams of crude India rubber cut in as small bits or shreds as pos- 
sible. Heat over boiling water 24 hours or let stand in the bath at 
60° ©. for three days. Pour off the clear melted paraffin and keep 
in the solid state. Use exactly as paraffin, clearing specimens for 
imbedding in xylol or toluol. With this method I have obtained 
many series of sections of all stages of Amblystoma as perfect as 
it is usual to obtain from ordinary objects. Obviously I could not 
use for this study preparations in which the yolk-bearing tissues 
were torn, or in which the yolk granules were scraped out of their 
position and driven along by the knife. In the figures given every yolk 
granule is faithfully drawn under the camera as accurately as pos- 
sible in its relations to the cell boundaries and nuclei. No yolk 
granule is omitted because it seems to be displaced. No drawings 
