368 EMBRYOLOGY 



to avoid rough treatment caused by upgrading the object too 

 quickly. The same remark apphes even more partieulai'ly to 

 clearing, which to get the best result should be done very 

 gradually. 



In order to bring out the outlines of blastoderm cells the living 

 ovum may be brought into \ per cent, solution of nitrate of silver. 

 After remaining there for half a minute to two minutes, according 

 to the age of the vesicle, it is brought into pure water and exposed 

 to the light. The preparations thus obtained are instructive, but 

 blacken rapidly, and cannot be permanently preserved. 



The blastodermic vesicle can be opened with fine needles, and 

 the blastoderm washed, stained, or impregnated with gold, and 

 mounted in glycerin or balsam. 



For embryonic areas and more advanced embryos, refer to 

 " Cytology," §§ 691 — 714. Kolliker recommends putting the 

 ovum into 0-5 per cent, solution of osmic acid until it has taken on 

 a somewhat dark tint, which happens in about an hour, and then 

 treating it with successive alcohols for several hours. If the ovum 

 be adherent to the uterine mucosa the portion of the membrane 

 to which it is fixed should be left, stretched out with pins, in 0-1 

 per cent, solution of osmic acid for from four to six hours. The 

 blastodermic vesicle can then easily be removed, and further 

 treated as before. For sections Kolliker fixes with osmic acid. 

 V. Beneden treats the ova for twenty-four hours with 1 per cent, 

 solution of chromic acid, then washes well, and brings them 

 through successive alcohols. Chromic acid has the advantage of 

 hardening thoroughly the vesicle, and maintaining at the same 

 time the epiblast cells perfectly adherent to the zona pellucida. 

 V. Beneden also recommends the liquid of Kleinenberg. 

 Henneguy writes that he frequently employs it for embryonic 

 areas and embryos of various ages, always with excellent 

 results, Fol's modification of the liquid of Flemming gives 

 excellent results. For staining, Henneguy recommends borax- 

 carmine, or Delafield's haematoxylin for small embryos ; for 

 large ones he found that his acetic acid alum-carmine was the 

 only reagent that would give a good stain in the mass. 



For sections imbed in paraffin, or double imbed. 



807. On the Fixation of Whole Tubes. This may be done in 

 Carnoy, Bouin or Helly. For rapidity of fixation, and faithfulness 

 of preservation of cell aggregates Carnoy 's fluid or preferably 

 Sansom's modification of Carnoy are to be recommended. Chrome- 

 formalin mixtures penetrate less readily, but often give fine results. 

 Bouin's fluid we have found capricious. On the whole we think 

 that warm Helly or Miiller-formol as a preliminary fixation are 

 to be recommended for small tubes. Regaud's or Schridde's 

 methods should give efficient fixation (§§ 699, 702). Many 

 workers have used the picric mixtures like picro-sulphuric and 



