The plates of this atlas are reproduced from photographs of the eggs of the sea-urchin, Toxopncustcs variegatus, Ag., procured at Beaufort, N.C. The eggs, carefully selected from ripe females, were artificially fertilized in seawater , and preserved at regular intervals. The eggs of this species have the great advantage of being devoid of pigment and very transparent, so that nuclei, asters, and spindles can be clearly seen and their general history followed in life. After testing many different fixing agents it was found that the best results were obtained by^^subhm^i^acedc_(8^parts concentrated aqueous solution of corrosive sublimate and 20 parts glacial acetic acid). When properly used, this reagent causes no chano-e of form and no shrinkage or distortion of the internal structures. The finest details of the nuclear and archoplasmic structures are shown with a clearness and brilliancy which far surpass the results of pure sublimate, Hermann's fluid, chromic acid, chrom-acetic, picro-osmic, picro-sublimate, and even Flemming's fluid, though the last-named reagent (weaker formula) gives good results which were used as a check on the sublimate-acetic. After fixation the eggs were preserved in alcohol, embedded in parafifine, sectioned in the usual manner, and stained on the slide by Heidenhain's iron-hxmatoxylin. The best results were obtained with sections from 3 to 5 /j, in thickness (_j^to Woo inch), stained twenty-four hours in the h^ematoxylin and differentiated in i per cent solution of iron-alum to a bright but delicate blue. The photographs (with the single exception of No. 29) were taken with Zeiss 2 mm. apochromatic oil-immersion, projection-eyepiece No. 4, at an enlargement of 950 to 1000 diameters, and are reproduced without reduction. Details regarding the photographic technique are given in a following note by Dr. Leaming. The only successful attempt hitherto made to show the early history of the ovum by means of photography is that of Van Beneden and Neyt * wlio published twenty-four photographic reproductions of the eggs of Ascaris mcgaloccphala showing many of the more important facts. Admirable as these figures are in many respects, they are nevertheless very defective in respect both to the completeness of the series and to clearness of detail. Their principal defects are due to the fact that the photographs were taken from entire eggs and not from sections. The blurring of images thus procured is so great as to render the photographs very unsatisfactory; and the author's experience has shown that clear and satisfactory negatives can only be taken from sections, and these must be as thin and as sharply stained as possible.
