No. 2.] THE SEA-URCHIN EGG. 473 



DESCRIPTION OF ILLUSTRATIONS. 



The accompanying photographic illustrations, enlarged from 950 to 1000 

 diameters, are from sections of the eggs of Toxopneustes variegatus, Ag., fixed in 

 sublimate acetic and stained with iron hsematoxylin (see p. 444). They are repro- 

 duced by the gelatine process, which is entirely mechanical, involving no hand- 

 work or retouching of the plates, and is therefore absolutely faithful to the 

 original negatives. The latter were taken by Dr. Edward Learning with the Zeiss 

 apochromatic oil-immersion, 2 mm., projection eye-piece 4. 



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