110 CHARLES R. STOCKARD 



it myself for ten years on Fundulus eggs, but have never before 

 succeeded in getting this hea\^ outline of the cells. It seems 

 scarcely possible that so striking an appearance could have 

 been overlooked, yet it is perfectly simple to obtain. Fundulus 

 yolk-sacs fixed in this way are equally as beautiful as silver 

 preparations of cell boundaries. 



In addition to the above, I have used for the first time another 

 solution which renders the specimen still more transparent. 

 This is a mixture of strong formalin 5 parts, glacial acetic 4 parts, 

 glycerine 6 parts, and distilled water 85 parts. Eggs are placed 

 directly into this and left for two days, and then transferred to 

 10 per cent formalin for permanent preservation. The fluid 

 mixture causes the egg to swell to some extent but it leaves the 

 yolk as clear as in life and by fixing the cells causes them to stand 

 out in beautiful contrast (figs. 5, 6, etc.). The mixture of glyc- 

 erine and glacial acetic has been used for a long time in prepar- 

 ing transparent specimens of invertebrate eggs. Wilson in 1892 

 used it with Nereis eggs. The proportions here employed have 

 been used by several students at Woods Hole and are not original 

 with me, except that others leave the eggs permanently in the 

 mixture while it seems better to put them in formalin after two 

 days. The eggs remain equally transparent in formaUn. 



The cleared specimens are most valuable for use in connection 

 with the studies of the living. But the remarkably beautiful 

 filamentous processes of the wandering mesenchyme cells and the 

 endothelial lining cells of early vessels are not so extensive as 

 during life. Some shrinkage or contraction of these processes 

 always accompanies fixation. The movement of the processes 

 in life also gives one a much better conception of their form and 

 structure. 



THE EARLY WANDERING CELLS 



About two hours after fertilization the eggs of Fundulus 

 heteroclitus have undergone the first division and are in the 

 two-cell stage. The cleavages then continue in a more or less 

 regular fashion to form a discoidal mass of cells as a cap on the 

 yolk. At eighteen to twenty hours the germinal disc is begin- 



