146 FOOT AND STROBELL. 



EXPLANATION OF PLATE A. 



The reproductions for the following plates were made by Edward Bierstadt of 

 New York. His work was so admirably done that neither strength nor definition 

 has been sacrificed by the process of reproduction. 



In order to economize space, only a small portion of the negatives of Nos. 

 1-5, 10, II, 13-15, has been reproduced. The eggs of Nos. 2-5, 10, were fixed 

 under a Zeiss 2 mm. immer., one of us applying the fixative while the other 

 watched its effects. All the photographs were taken with Zeiss 2 mm. immer. 

 projection ocular 4. 



Photo, i. Living egg (stage, telophase of first cleavage spindle). Periphery of 

 egg, showing part of a polar ring, surrounded by sap globules. Egg slightly col- 

 ored with weak methyl green. The negative of this egg being a little sharper, it 

 was chosen in place of one of an unstained egg of the same stage. In these cases 

 the light was transmitted through a part of the egg fully 100 /i thick; thus a very 

 sharp negative could scarcely be expected. X 500. Medium, distilled water. 



Photo. 2. Periphery of a segmenting egg, after 20 minutes in corrosive sub- 

 limate. Delafield haematoxylin. Before killing the egg, many of the sap globules 

 were sketched with the camera lucida, including the line of five on the right. The 

 fixative produced no perceptible change in their size or contour. {Cf. No. 5.) 

 X 500. Medium, distilled water. 



Photo. 3. Periphery of oocyte, second order, after chromo-acetic (15 mm.). 

 Stained with alum cochineal, hardened in alcohol and cleared in xylol. The 

 small sap globules have fused, forming irregular patches. X 500. Medium, 

 xylol. 



Photo. 4. Periphery of egg just after formation of second polar body. Slightly 

 flattened. Killed in .1 % osmic acid (15 mm.). Gentian violet. X 500. Medium, 

 distilled water. Cf. size of sap globules with those of egg of a little later stage 

 (No. 5). 



Photo. 5. Periphery of egg at pronuclear stage. Pronuclei one-half maximum 

 growth. Egg pressed until the cytoplasm reached the outer membrane. This 

 was done under a 2 mm. immer., and the sap globules appeared neither broken nor 

 fused by the gentle pressure. A comparison of the photograph with a living egg 

 at the same stage shows them to be the normal size. 1% osmic acid, i hour. 

 Unstained. X 500. Medium, distilled water. The sharp black specks are osmo- 

 phile granules ; those out of focus appear as faint rings. 



Photo. 6. Section (4 it) through cytoplasm and one polar-ring of egg at 

 telophase of first cleavage spindle. Fixative, chromo-acetic. Hardened in 

 40 % formaldehyde, 26 hours. Stain, iron haematoxylin. X 340. Medium, xylol 

 balsam. 



