xvi THE HISTORY OF ZOOLOGY (;87 



lowly organisms which occur in putrefying substances, do not arise 

 de novo, but are the product of germs in the floating dust of 

 the air by the exclusion of which ])iitre faction may be absolutely 

 prevented. 



During the last quarter uf a century the progress of ZoiAogy 

 has been profoundh' influenced by the improvements in micro- 

 scopical methods, especially by the invention and perfection of 

 the microtome, the method of serial section-cutting, and the 

 various ways of preserving, imbedding, and staining tissues. The 

 microtome began as a simple contrivance for holding small objects 

 firmly while sections of them were cut by hand with a razor or 

 other knife, and has developed into the various modern forms of 

 the instrument in which the knife is fixed in a plane parallel to 

 the surface of the object, and the latter is raised mechanically by 

 small and equal increments as the sections are cut. In this way 

 perfectly regular sections are obtained of an even thickness not 

 exceeding tlie diameter of a cell. The method of imbedding 

 began by simply holding an object, too small or too soft to be 

 grasped by the fingers, between two pieces of carrot or pith, and 

 has gradually been evolved into the present method of complete 

 impregnation with paraffin or celloidin, by means of which 

 imbedding material and object form a homogeneous mass. 

 Simple preservation in alcohol has given place to elaborate fixing 

 methods by means of chromic, picric, or osmic acids, 

 platinum chloride, corrosive sublimate, etc., and gradual hardening 

 in alcohols of increasing strength. Similarly, direct staining with 

 an ammoniacal solution of carmine has developed into innumerable 

 methods of differential staining, mostly with aniline dyes, by 

 which the various tissues and the constituents of the cell — 

 chromatin, centrosomes, etc. — are clearly brought into view. By 

 the serial methods successive sections of an embryo or small 

 animal are mounted in regular order, so that the organs, tissues, 

 etc., can be traced through the series. In this way the 

 dislocation of parts produced by dissection is avoided, organs are 

 seen in absolutely natural relations, and parts quite undiscernible 

 either by dissection or by microscopic examination of the whole 

 animal or of dissociated parts of it, are clearly brought into view : 

 the study of the structure upon which the sections are intended 

 to throw light may be further facilitated by the fashioning of 

 models in wax, reconstructed by putting together reproductions of 

 the sections enlarged to scale. Morphological inquiry has, in fact, 

 been brought within measureable distance of a precision limited 

 only by the imperfections of our eyes and optical instruments. 

 Similar accuracy in the topographical anatomy of the larger 

 Animals, including Man, has been attained by freezing the whole 

 subject and cutting it into sections with a saw. 



These improved methods have necessitated a re-examination by 



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