vi PREFACE. 



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 sea-water, 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 archo- 

 plasmic 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. 



The set of photographs here published is selected from a set of nearly two hundred negatives. The series is 

 incomplete in some particulars and the figures vary considerably in effectiveness; but as circumstances render it 

 impracticable at present to publish a larger number, the series is put forth as it stands, in the hope that it may 

 serve a useful purpose. In order to render the work more useful for students an introduction has been prepared 

 in which the more important facts are briefly reviewed from a general point of view. A series of explanatory text- 

 figures are also given as a key to the phototypes. These figures are from camera lucida drawings of the object 

 (in many cases the same specimen photographed), and although necessarily slightly schematic, they are in no sense 

 mere diagrams (except in a few cases, as stated in the explanations), but are attempts to represent as closely as 

 possible the actual appearance of the object. 



I am glad to express my great obligations to Dr. Leaming, to whose skill the excellence of the photographs 



is largely due and without whose cooperation the present publication of this atlas would have been impossible. My 



thanks are also due to Mr. Bierstadt for the especial pains he has taken with the reproduction of the plates. It 



should be added that the process is a purely mechanical one, which involves no retouching of the plates, and the 



defects of the original preparations appear in the figures without disguise. 



EDMUND B. WILSON, 



CoLUMBL* College, May, 1895. 



* Bulletins de Tacadcfniie royale de Belgique. 3 serie, June 14, 1887. 



