74 FIXATION 



The report on the kidney-cortex of the mouse may be made 

 under these headings: — 



spaces between convoluted tubules (as above, under semini- 

 ferous tubules) ; 



cytoplasm of convoluted tubules (as above, under spermato- 

 genetic cells) ; 



free border of epithelium of first convoluted tubules (whether 

 smooth or ragged); 



nuclei (whether smoothly rounded or distorted) ; 



red blood corpuscles (whether they retain their natural shape or 

 are swollen into spheres or otherwise distorted) ; 



dyeing (as above). 



The most delicate parts of the two organs are the cytoplasm of 

 the primary spermatocytes and the free border of the first con- 

 voluted tubules. Really good fixation of these is rarely seen in 

 paraffin sections. 



It is convenient to compare fixatives by assigning them to differ- 

 ent grades. It is an arbitrary matter to decide the number of such 

 grades, but everyone who undertakes work of this kind will agree 

 that two are too few (because more accurate distinctions can be 

 made with confidence) and ten too many (because, if the assessment 

 were repeated with the same slides, it would often happen that the 

 same preparation was not assigned to the same grade). It is perhaps 

 reasonable to make five (grade I the best, grade V the worst). To 

 prevent waste of time from prolonged indecision, one may some- 

 times record the result as I-II, II-III, etc. Examples of grade I 

 and grade V fixation are shown in fig. 9. Some of the results of this 

 test with simple fixatives and mixtures will be mentioned in 

 chapters 5, 6, and 7. 



The fact that a fixative falls into a low grade in this test by no 

 means condemns it. Grade V fixatives are usable with many organs 

 that are less delicate than mammalian testis and kidney-cortex. 

 They may also have particular virtues of their own. Altmann gives 

 blocks of tissue that crack easily with paraffin embedding; there 

 is considerable shrinkage and distortion of the cells ; chromatin is 

 dissolved away. Mitochondria, however, are excellently fixed, and 

 Altmann remains a useful fluid despite its manifest defects. (The 

 cracking does not occur when Altmann tissue is embedded in 

 collodion.) As a general rule, nevertheless, it is obviously best to 



