MUSEUM OF COMPARATIVE ZOOLOGY. 65 
tic appearance, and therefore can be used only in connection with other 
methods. 
Corrosive sublimate, either cold or hot, renders the eggs too brittle. 
On account of the thickness of the chorion neither chromic acid nor 
acid alcohol can be entirely extracted, and osmic acid will not penetrate. 
Borax carmine (Grenacher’s alcoholic) has proved to be, on the whole, 
the best staining fluid. It is difficult to make any stain penetrate the 
material of the later embryonic stages and those subsequent to hatch- 
ing, on account of the development of the cuticula. This difficulty was 
at length overcome by prolonged immersion in the staining fluid. In 
some cases seventy-two hours were required to obtain an adequate stain. 
Owing to the weak grade of alcohol used in making the stain, the eggs, 
to prevent maceration, were left in the stain only twenty-four hours at a 
time. They were then re-hardened, and after an interval immersed again 
in the staining fluid. 
The brittleness of the yolk of spiders’ eggs constantly produces crum- 
bling of the sections. I have found that the yolk of eggs treated with 
Perenyi’s fluid may be cut satisfactorily ; in other cases I have used suc- 
cessfully Mason’s collodion method.* 
Composition of the Egg.—The composition of the freshly deposited 
egg has already been described with accuracy in most particulars by 
Balbiani and others. In certain points, however, there has been neither 
agreement in descriptions nor great accuracy. 
To make clear the subsequent account I shall describe briefly the con- 
stituent parts of the egg. It is enveloped by two membranes in contact 
with each other. The outer, or so-called chorion, is tough and homo- 
geneous, with its external surface covered by granules, which vary in 
size and abundance in eggs of different species. In Agelena nevia they 
are arranged in a single layer, and do not offer any serious impediment 
to observations ; in some species (e. g. Lpetra diadema), however, they 
are several layers deep as well as very large, and must be removed to 
allow accurate observations. On removing these granules the chorion 
presents a finely punctate appearance, which is perhaps due, as Balbiani 
has suggested, to the impressions left by the granules. This membrane, 
unlike the chorion of insects, is added to the egg while it is passing 
through the oviduct, and like the egg-shell of Apus would fall into the 
eategory of “secondary egg membranes,” as defined by Ludwig (’74). 
* See E. L. Mark, “ Notes on Section Cutting,” in the American Naturalist, 
June, 1885, p. 628. 
VOL. X1I.—NO, 3. 
