120 Animal Micrology 



utes in a 0.3 per cent, aqueous solution of osmic acid, stained in picro- 

 carmine, and transferred to a mixture of glycerin and water, equal parts. 

 They should remain in this fluid for a week under a bell-jar so that the 

 water gradually evaporates. The object may then be mounted in 

 formic -glycerin (formic acid 1 part, glycerin 99 parts). To avoid pressure 

 of the cover-glass, the object should be mounted in a cell or between two 

 slips of paper or pieces of cover-glass. If the preparation is to be per- 

 manent the cover-glass should be sealed (see chap, xiii, II, A, 6). 



To render the cell outlines distinct stages of from 70 to 80 hours are 

 best treated, after rinsing in distilled water, with a 1 per cent, aqueous 

 solution of silver nitrate for 3 minutes and then exposed to light in a 

 dish of distilled water until they become brown. They are then treated 

 with water and glycerin and mounted in formic-glycerin as in the case of 

 younger stages. 



For sections, the embryos should be placed in Hermann's (reagent 26, 

 Appendix B) or Zenker's (reagent 6) fluid for about an hour, then washed 

 in the customary way for these methods, stained in borax-carmine or 

 alum-cochineal, and sectioned in paraffin. 



In opening the uterus, the incision should always be made along the 

 middle of the free side, opposite the insertion of the peritoneal fold, 

 because this line of insertion marks the region of attachment of the 

 embryo within the oviduct. By the seventh or eighth day the developing 

 ova have taken up positions at intervals along the inner walls of the 

 uterus and have become so firmly attached to the mucous membrane 

 that they can no longer be detached unmutilated. For further particu- 

 lars regarding the embryology of the rabbit, the reader is referred to 

 E. Van Beneden and Charles Julin's " Recherches sur la formation des 

 annexes foetalis chez les mammiferes," Archives de Biologic, Vol. V 

 (1884), p. 378. 



7. For Older Stages of the Mammalian Embryo, pig embryos are com- 

 monly employed. They may often be procured in large numbers and 

 with little trouble at the larger pork-packing establishments. The most 

 valuable stage for study is an embryo of from 10 to 13 mm. in length. 

 In most laboratories it is customary to make a detailed study of an 

 embryo of about this stage and then a more general survey of both 

 smaller and larger sizes. 



Early stages are much more difficult to obtain than advanced stages. 

 Embryos of 6 mm. length and over may usually be readily located by 

 the enlargements which they cause in the uterine walls. The uterus 

 should be handled carefully and opened as soon as possible. The 

 embryo is best removed by means of fine forceps and a horn spoon. It is 

 very delicate and should not be handled roughly. The chances are that 



