28 CYTOLOGICAL TECHNIQUE 



readily to quantitative analysis than do the other intrinsic arti- 

 facts caused by fixatives. Interesting experiments can be made 

 with the same gelatine-albumin gel as is used in studies of rate of 

 penetration. The results are shown in fig. 5. Acetic acid swells 

 strongly, and indeed this is one of the main reasons why it is 

 included in so many fixative mixtures, for the swelling counter- 

 acts the shrinkage caused subsequently by embedding, espec- 

 ially if the swollen cells are fixed in their swollen state by a 

 coagulant fixative (see p. 62). Fig. 5 shows that most of the 

 primary fixatives do not alter the volume of the gel greatly, but 

 ethanol shrinks it strongly. 



Whole animals or whole organs may be measured before and 

 after fixation, and at various stages in the subsequent processes of 

 microtechnique; or cubes may be cut out of large organs and 

 treated in the same way. The most interesting results, however, 

 are those obtained by the study of single cells. It is best to choose 

 loose, spherical cells for this purpose, since they are simplest 

 from the geometrical point of view. The eggs of echinoids are 

 suitable in this respect. Some results are shown in Table 1. 



TABLE 1 



The volumes of unfertilized eggs of the echinoid, Arbacia pustulosa, at 

 various stages of preparation for microscopical study, expressed as 

 percentages of the volumes while alive in sea-water. Each figure is 

 calculated from the mean diameter of 10 to 15 eggs. {Data of Hertwig^^) 



Comparable results are shown by studies of the spermatocytes 

 of the common snail. Helix aspersa.^^^ When they have been 

 fixed in formaldehyde solution, embedded in paraffin, sectioned, 



