HOSKINS: LABORATORY METHODS IN EMBRYOLOGY. 179 



vascularity ; also, in case of those containino; embryos of 

 greater length than 5 or 6 mm., by the series of enlargements 

 that contain the latter. These uteri are selected and carried 

 to the table for examination. Each of the previously men- 

 tioned enlargements is then slit open with the scissors. This 

 will also slit the false amnion, thus opening the vesicle in 

 which floats, in a copious supply of fluid, the true amnion 

 and its contained embryo. This is then seized and the false 

 amnion, to which it is attached, severed with the scissors. 

 For handling the embryos at this stage of the manipulations, 

 fingers are preferable to any sort of forceps. After a suffi- 

 cient number of embryos have been obtained and placed in 

 the fixative, they are taken to the laboratory and, after fixa- 

 tion is completed, washed, sorted, and preserved in eighty 

 per cent, alcohol until needed. 



As in case of the chicks, the pig embryos to be sectioned 

 should be given a preliminary stain with eosin before they 

 are embedded. They are then sectioned at a thickness of from 

 10 to 20 microns, depending upon the size of the embryo. 

 The sections are mounted on slides and the eosin removed. 

 For staining, the most satisfactory results have been obtained 

 by the following hsemalum-cochineal mixture : Equal parts 

 of Minot's alum-cochineal and Mayer's hiemalum ; this is 

 followed by a counterstain of orange G. This method can be 

 used with material fixed either in Zenker or picro-sulfuric. 

 With this combination there is very little danger of over- 

 staining ; the cochineal has a retarding effect on the htpm- 

 alum, and finally so impregnates the tissue as to prevent 

 overstaining with the latter. An excess of orange G can be 

 easily corrected by an alcohol bath. This combination gives 

 exceptionally good differentiation ; the cochineal is selective 

 for nervous tissue and brings out the various elements in the 

 central nervous system particularly well, while the hsemalum 

 gives a very clear nuclear stain. The orange G marks off 

 distinctly both cell and basement membranes, as well as per- 

 ipheral nerve fibers. 



Each mounted embryo that is added to the collection is 

 given an accession number. This is done whether it is put 

 up as an exhibition specimen, either whole or dissected, or 



