HOSKINS: LABORATORY METHODS IN EMBRYOLOGY. 177 



desired amount of the fetal membranes may be included. 

 The disc is now removed on a section-lifter to a Petri dish of 

 water. By gentle manipulations the adherent yolk and the 

 vitelline membrane can be floated loose from the embryo. 

 In case they prove refractory a gentle agitation of the water 

 by a pipette will usually dispose of the difficulty. At this 

 stage great care is necessary. 



For convenience in further manipulation it is advisable to 

 harden the embryos, particularly if they are to be sectioned. 

 They are carefully flattened in a vessel with a plane bottom 

 — preferably a Petri dish — then treated with the hardening 

 fluid — ten per cent, formalin saturated with mercuric chlorid. 

 Only a few drops should be used, and this carefully applied, 

 in order to avoid any folding or creasing of the blastodisc or 

 membranes. After a treatment of from five to thirty minutes, 

 the embryo is washed in running water from fifteen minutes 

 to two hours — the length of time depending on the size. This 

 step follows immediately after fixation, in case the hardener 

 is not used. 



For staining surface mounts for general microscopic 

 study, Grenacher's alum-carmine is very satisfactory, but if 

 the preparation is to be used with a projection lantern, borax- 

 carmine is slightly better. For embryos that are to be sec- 

 tioned, either of these may be used as an in toto stain, but it 

 is often advisable to use others, staining on the slide. In 

 this case the chick should be given an in toto stain with some 

 such color as eosin ; this makes it more easily seen, and hence 

 less likely to injury in the manipulations of sectioning and 

 mounting. This stain is later removed from the sections 

 with alcohol and some other stain substituted. Mayer's 

 h^emalum is perhaps the best for general purposes. Older 

 stages should be stained very lightly. For clearing whole 

 embryo preparations pure cassia oil has been found much 

 superior to anything else ; it causes very little shrinkage and 

 does not make the material brittle, as do most other clearing 

 agents. 



To secure pig material in any considerable amount, it is 

 necessary to visit a large pork-packing establishment. In 

 any of these it is easy to secure embryos of any size from 



