2O8 EDWIN G. CONK.LIN. 



publications (Foot and Strobell, 1899, 1900, 1901, 1902) is, in 

 some respects, the most satisfactory ever devised and yet it is as 

 simple as it is complete. The distinguishing feature of this 

 method consists in the accuracy and rapidity with which an exact 

 focus can be obtained ; this fact will be better appreciated when 

 the photographs which illustrate this paper are examined in 

 detail and when it is said that thirty such photographs, all of 

 them satisfactory, were taken in less than four hours. With 

 more time and material a more complete series of stages might 

 have been photographed, but this series is sufficiently complete 

 to show the principal features in the differentiation and localiza- 

 tion of the egg substances. 



All of the photographs are of living eggs in sea water and were 

 taken with a Zeiss Apochromat Obj. 16 mm., Compensating Oc. 

 4, the bellows length being sufficient to make the magnification 

 1 1 2 diameters. The photographs have not been reduced in the 

 process of reproduction and neither the negatives nor prints zivvr 

 retouched or altered in any respect u'haterer. 



The eggs photographed were artificially fertilized and the earli- 

 est stage shown (Photo i) was taken about three quarters of an 

 hour after the sperm were mixed with the ova, but not more than 

 fifteen minutes after the entrance of the spermatozoon into this par- 

 ticular egg. The method of procedure was to place a large drop 

 of sea water containing a considerable number of eggs on a slide 

 and cover with a glass supported by glass "feet" i/o/J. thick 

 (the eggs are about I 50 , in diameter). Suitable eggs were then 

 selected and photographed by daylight, the exposure ranging 

 from ten seconds to one minute ; the shorter exposure was found 

 to be sufficient while the longer was greatly overtimed (Photos 

 i and 13). In order to obtain good color values it is necessary to 

 have the diaphragm rather widely open and this renders the defini- 

 tion somewhat less distinct than it otherwise would be. Further- 

 more a low magnification was necessary in order to secure sufficient 

 depth of focus to show the entire egg ; even with the power em- 

 ployed it was not possible to bring the whole of the egg into 

 good focus at one time. In spite of these evident disadvantages 

 the photographs are really remarkable. Few, if any, other eggs 

 are known in which the organization is so notable as in Cynthia 



