160 ROBERT T. HANCE 



house ill which the pigs were obtained was rather slow and the 

 embryos were probably dead before they reached me. 



Since the work on sectioned material was completed I have 

 obtained another lot from Cincinnati. These specimens I pre- 

 served both in Flemming's weak and strong solutions to which 

 urea had been added. The fluids were used at a temperature of 

 four degrees Centigrade. The embryos apparently were not in 

 a cycle of division for figures were too few to justify study. The 

 amnion from the same pigs was fixed separately and mounted 

 without sectioning. The amnion is quite thin and gave excel- 

 lent results with the certainty of uncut cells. The slight shrink- 

 age which occurs during the infiltration of paraffin is avoided and 

 the chromosomes appear slightly larger in the amnionic material 

 and in general, better separated. No differences were noted in 

 the character of the fixation by either killing fluid. Both gave 

 excellent results as may be judged from the photomicrographs, 

 figures 88, 89, 90 and 93. I cannot say, at present, whether the 

 weak solution will work equally well on thicker tissue. The 

 chief advantage of the weak solution, as far as known now, is 

 one of cost. 



Counting and checking 



When as large a number of chromosomes are involved in as 

 small a space as are the chromosomes in the cells of the pig, 

 drawing and counting is not easy, notwithstanding the almost 

 perfect separation of the indi\'idual elements. Inaccuracies in 

 drawing were avoided in the following manner: The chromosomes 

 of a cell were carefully drawn on a 3 x 5 card and the drawing 

 was then checked with oculars of various powers. The card was 

 then filed away for a time. Later, it was brought out and on it 

 the same set of chromosomes was again drawn. This drawing 

 w^as checked as in the case of the first. Then the two drawings 

 were compared and the points of difference, should any be found, 

 were decided by comparison with the cell under the microscope. 

 This method saved considerable time and, I believe, gave very 

 accurate results. 



