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1885.] Microscopy. 1135 
Merkel’s fluid (platinum chloride, 1% per cent, and chromic acid, 
I% per cent, in equal parts), in which they are allowed to remain 
one and a half hours. They are next washed in flowing water 
for the same length of time, then treated with 50 per cent and 
70 per cent alcohol. They need remain only a short time in 
the first grade of alcohol (about thirty minutes), but should be 
left for twelve to twenty-four hours in the second. For stain- 
ing, I have used Grenacher’s alcoholic borax-carmine, adding 
to it from one-third to one-half its volume of glycerine. The 
glycerine intensifies the action of the dye,so that a moderately 
deep stain is taken in the course of twenty-four hours. 
It should be mentioned that it is best to stain immediately 
after the eggs have remained the required time in alcohol, as 
receptivity for the staining fluid diminishes considerably with the 
lapse of time. The osmic acid has time to penetrate to all parts 
of the embryo, and the blackening is arrested and partially re- 
moved by the action of Merkel’s fluid. The differential effects of 
the osmic acid are, however, sharpened under the influence of the 
chrom-platinum solution. 
This method has enabled me to trace out the history of the 
entoderm, and the precise origin of the nerve-chord, nephridia, 
salivary glands, larval glands, &c. The results of my study will 
be published in full later, and I shall here only give the more im- 
portant conclusions. Each of the germ-bands,as I have shown 
elsewhere, is made up of three distinct layers, namely: (1) an 
epidermal layer; (2) a layer consisting of four longitudinal rows 
of cells, and (3) a deeper layer, next to the yolk, composed of 
larger cells. 
The four rows of cells, forming the layer subjacent to the epi- 
dermis, are the products of four larger cells at the posterior end 
of each germ-band. These four cells are called “wxeurod/aszs” in 
the papers referred to. The innermost layer, which is wholly 
mesoblastic, is derived from alarge “ mesoblast,” which lies below 
the four “ neuroblasts.” Thus the two deeper layers of each germ- 
band is made up of the products of five cells. 
The main point to be determined was the precise origin of the 
nerve-chord. In my first paper dealing with this subject I was — 
unable to settle this point satisfactorily. I satisfied myself that 
the nerve-chain was formed from the products of the so-called 
“ neuroblasts,” but I was mistaken in supposing that aX of these 
products entered into its composition. 
The method above given has shown that of the eight rows of 
these cells (four in each band), only the two median ones give rise 
to the nerve-chord. The lateral row of each band probably gives 
origin to muscular elements, while the two rows, lying between 
the median and the lateral rows, furnish the basis forthe nephridial 
1Quart. Jour. Mic. Sc., 1878, and Zool. Anz., No. 1. 
