ON EGGS OF MARINE ANIMALS 59 



cells are successfully fixed by such a fixing solution only if the 

 object is of a much smaller size than that used in fixation by 

 the more generally employed agents. Gut cells of mammals, 

 kidney cells of various vertebrates may be most beautifully fixed 

 by solutions containing osmic acid, provided the cells are 

 removed in strips or isolated into their constituent layers: 

 mucosa cells of the gut and a small length of kidney tubules 

 separated from contiguous layers give perfect fixation, whereas 

 a block of cells equal in bulk to the strip does not. One factor 

 in fixation with solutions containing osmic acid is the presence 

 of fat or yolk. If the tissues to be studied are rich in these, the 

 difficulty of fixation is increased. But even this is not a serious 

 obstacle, if one takes small enough pieces. This fact is shown 

 in a simple way. Of an emulsification of acid and olive oil with 

 egg yolk as emulsifying agent, mayonnaise, I have fixed indi- 

 vidual drops of successive diminution in size. The osmic acid 

 blackens each drop; but in the case of the larger drops it does 

 not penetrate even after weeks; cutting into these revealed the 

 interior as yellow as the original mayonnaise from which the 

 drops came. The smaller drops, on the other hand, those 

 ranging in size from 40 to 200 microns in diameter, are blackened 

 throughout in less than an hour. Such droplets may be treated 

 as though they were eggs, that is, sectioned and stained. 



The difficulty of over-osmification, where this obtains, can 

 be overcome by not allowing the object to remain too long in 

 the fixing solution. In my experience, however, the problem 

 of over-osmification is often caused by a too great amount of 

 the acid in the solution as employed. If the proper amount of 

 osmic acid is used, cells do not show over-osmification even if 

 they remain in the fixative for weeks. I should point out that 

 often tubes contain more osmic acid than the weight named on 

 the label. I suggest that one use 4, 3, 2, and i cc. of this acid 

 made up in 2 per cent, solution. It is perhaps also true that 

 workers experience difficulty with the use of osmic acid because 

 they do not wash it out sufficiently. 



Against these shortcomings of osmic acid stands a compen- 

 sation: the ease in handling eggs having been fixed by it. It is a 

 great nuisance to collect eggs which have been fixed in solutions 



