Whole Moinits 379 



supported (page 381) to protect the delicate embryo from destructive 

 pressure. For sections, after washing the embryos can be dehydrated, 

 cleared and embedded in paraffin. 



Incubation time and determination of age can be puzzling. A per- 

 fectly aged chick sometimes is difficult to obtain, because the age is 

 determined not only by the number of hours in, and the temperature 

 of, the incubator, but the length of time the egg was in the hen and 

 the temperature at which the egg was stored. The temperature of the 

 incubator should be 37.5 °C and it usually requires about 4 hours to 

 Avarm up the egg once it is placed in the incubator. In other \vords, 

 a 36 hour chick would require about 40 hours in the incubator for 

 proper development. The accompanying chart (Figure 27) indicates how 

 coiuiting the number of pairs of somites in the chick will determine its 

 age; thus 10 pairs indicates an age of 30 hours. This is the best criterion 

 of development, rather than the number of hours in the incubator. 



Any bird or reptilian embryo can be fixed as outlined for the chick. 

 Airiphibian larvae can be dropped directly into fixative, after first dis- 

 secting off any surrounding yolk or jelly. 



Mammalian embryos should be dissected from the uteri and fixed in 

 Bouin's. If the uterine site also is to be preserved, cut through the uterus 

 on either side of the embryo and fix. A little fixative can be injected into 

 the uterine cavity to quicken the fixing action. 



Ascaris uteri are fixed in alcoholic Bouin's; sometimes Carnoy will 

 give desired results. 



Whole Mounts 



Whole mounts are slide mounts of entire specimens small enough to 

 be studied in toto and mounted in resins in toluene or water, in gimis 

 or glycerine as aqueous mounts, or as dry moinits. Some examples from 

 the innumerable methods will follow, with the hope that a technician 

 can adapt them to almost any whole-mount venture. 



fixation: 95% alcohol, followed by bleaching if necessary. From 70% 

 alcohol into bleaching fluid (3-4 drops of Clorox in 10 ml. 70% al- 

 cohol, or water plus H2O0 (1:1) with a trace of ammonia): 24 hours. 



procedure: 



1. Wash with 70% alcohol, 3 changes. 



2. Dehydrate in 95% and absolute alcohol. 



3. Clearing is often difficult. Try mixture of zylene and beechwood 



