468 



THE AMERICAN 



ATURALIST [Vol. XLTI 



ing and sealing the egg Miss Peebles' 4 method was fol- 

 lowed. For destroying the somites two fine depilatory 

 needles, gronnd to a hair point on an oil-stone, were used, 

 one, at the negative electrode, touching the albumen, the 

 other, at the positive electrode, serving to prick the 

 somite which was to be marked or destroyed. For the 

 current four Samson dry-battery cells, each with an 

 electromotive force of 1.5 volts, were connected in series. 

 To prevent infection the instruments were sterilized in 

 a flame. With this method of disinfection, 15 out of 84 

 embryos, or 18 per cent., were lost, but as the loss was 

 occasioned by the sticking of the blastoderm to the shell, 

 it can not be stated that it was not due in part to causes 

 other than bacterial infection. A Zeiss dissecting stand 

 was used for the operations, with lenses magnifying six 

 diameters, and whenever possible the work was done with 

 the bright sunlight shining in upon the blastoderm. So 

 great is the variation in distinctness of embryos at this 

 early age, that even with the best of light the somites 

 could not, except in a comparatively small number of 

 cases, be counted with certainty. In the 

 embryos, however, which were distinct, 

 there was no room for doubt as to their 

 exact condition at the time of the opera- 

 ~c tion. 



From several experiments, the results 

 of which furnish evidence for the solution 

 of the problem, the following case has 

 been selected for description : 

 sketc^madTat^time Number 50 was operated upon after 30 

 of operation. <• = hours of incubation. The operation was 

 place of operation. p er f orme( j w ffii the sun shining in upon 

 the blastoderm, the embryo was distinct, and its three 

 somites were readily counted. Fig. 1 is a sketch made 

 at the time, showing the place of the operation, in 

 which, it was noted, the needle passed obliquely inward. 



