MICHIGAN ACADEMY OF SCIENCE. 



THE PKEPAKATIOX OF SERIAL SECTIONS OF FROG 



EMBRYOS. 



BY BERTRAM G. SMITH. 



The developmental history of the frog- is one of the best with 

 which to illustrate the fuudamentals of vertebrate eiiibr.volo<>y. 

 Particularly is this true of the early stages, since cleavage and 

 gastrulation proceed in a manner which is presumably more primi- 

 tive, and certainly more easily observed, than the same phases of 

 develo])ment of the chick. But the difficulty of preparing serial 

 sections of these early stages of the frog- embryo has discouraged 

 many from using this material for class w^ork. 



In attempting to prepare serial sections of frog embryos in early 

 stages, fixed in Tellyesnicky's solution,^ the writer had trouble 

 in cutting paraffin in sections on account of the crumbling of the 

 yolk. Experience with other amphibian eggs led to the hope that 

 a remedy might be found in proper fixation of the eggs and careful 

 attention to some other details of paraffin technique, rather than 

 in resorting to the tedious method of celloidin technique. 



FIXING. 



It is well known that fonnalin is a good fixing and ]»reserving 

 solution for embryos of the frog" intended for surface study only., 

 and some have reported fairly satisfactoi\y histological results from 

 embryos, in late stages, fixed in rather strong formalin. I sectioned 

 some embryos, in blastula and gastrula stages, fixed in formalin 

 (10% to 20%) only. When imbedded in paraffin, they cut like wax, 

 in fairly thin sections, at ordinary room temperature. The form of 

 the embryo is preserved perfectly. On the other hand, the cytologi- 

 cal details were not good; numerous small fissures extended in all 

 directions irrespective of cell walls and it was difficult to determine 

 the outlines of individual cells; the germ layers were not well 

 differentiated. 



The results suggested that a combination of fonnalin with the 

 bichromate-acetic solution might prove successful. T therefore used 

 the bichromate-acetic-formalin solution that had already proved well 



1. The solution is made up as follows: 



I'otassium liiohromatc 2g. 



Glacial acetic acid .'ice. 



Water 95cc. 



