108 CHARLES R. STOCKARD 



all be seen to form in the living specimens. The association of 

 the four types is also clearly determined. In addition to these 

 cells the large periblast nuclei are conspicuously seen. The 

 periblast never gives rise to any type of tissue or cells, but finally 

 the nuclei become swollen and distorted and degenerate in the 

 manner Wenckebach long ago described. 



MATERIAL AND METHODS OF STUDY 



The material used for these observations and experiments 

 was the embryo of the Teleost, Fundulus heteroclitus. The 

 eggs of this fish are very transparent and may be readily observed 

 by transmitted light with a high power microscope. A compound 

 binocular microscope made by E. Leitz has been found to serve 

 splendidly for such observations since with this instrument an 

 oil immersion lens may be used to study the embryo suspended 

 in a hanging drop from a thin cover glass over a hollow slide. 

 A double or triple lens condenser facilitates the regulation of 

 light and in a darkened field the almost transparent cells may 

 be seen while granular or pigmented cells are distinctly outlined. 



The mesenchyme cells are of sufficient size to be readily fol- 

 lowed with a Zeiss DD objective while the eggs are grouped on 

 the bottom of a watch glass. An ordinary microscope serves 

 almost equally well for these observations but the Leitz binocular 

 has the advantage of producing an apparently stereoscopic effect, 

 since it permits the observer to look with both eyes at the same 

 time. One is also enabled to see much better and look much 

 more continuously than with one eye. An ordinary binocular 

 microscope is unfit for the finer observations on account of the 

 poor arrangements for condensing the light, and the magnifi- 

 cation is insufficient for details of structure. 



Observations have been made on the normal embryo at all 

 developmental stages. Specimens in which the circulation of 

 the blood was never established were also used, since in these the 

 behavior and development of the cells of the yolk-sac are in no 

 way contaminated by the introduction of additional cells brought 

 in the circulating current. Such specimens enable one to hold 

 the yolk-sac in its original condition so far as cellular elements 



