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 ci am- 

 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 diff'erent species. In Agelena noevia they 

 are arranged in a single layer, and do not offer any serious impediment 

 to observations ; in some species (e. g. Epeira 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 i<" is passing 

 through the oviduct, and like the egg-sheU of Apus would fall into the 

 category 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. XII. — NO. 3. 



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