16 TEXT-BOOK OF EMBRYOLOGY. 



and cut in either celloidin or paraffin, stained in haematoxylin and eosin (see Appendix) and 

 mounted in xylol-damar. 



The frog's eggs are good examples of telolecithal ova with a definite but not complete 

 polar differentiation. They may be collected in ponds in the early spring and preserved 

 indefinitely in 5 per cent, formalin. For gross study a few of the ova may be separated 

 from the general mass and examined under a hand lens. For internal structure, the ova 

 may be carefully removed from the gelatinous mass, stained in toto with borax carmin, 

 embedded in celloidin or paraffin, sectioned, and mounted in damar. Ova fixed in any 

 osmic acid mixture will show the yolk granules stained black. 



The various parts of a hen's egg (telolecithal ovum with extreme polar differentiation) 

 may be observed by simply removing a part of the shell. To study the egg, however, with- 

 out the secondary membranes, it must be seen in the hen's ovary. 



References for Further Study. 



CONKLIN, E. G.: Organ-forming Substances in the Eggs of Ascidians. Biol. Bull., 

 Vol. VIII, 1905. 



WALDEYER, W.: In Herwig's Handbuch der vergleichenden u. experimentellen Entwick- 

 elungslehre der Wirbeltiere. Bd. I, Teil I, 1903. Also contains extensive bibliography. 



WILSON, E. B.: The Cell in Development and Inheritance. 2d Ed., 1900. 



