xvi THE HISTORY OF ZOOLOGY 647 



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 method successive sections of an embryo or small 

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

 etc., can be traced throughout 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. 

 Morphological inquiry has, in fact, been brought within measurable 

 distance of a precision limited only by the imperfections of our 

 eyes and optical instruments. Similar accuracy in the topo- 

 graphical 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 

 their aid of every group in the animal kingdom, and, as a result, 

 our knowledge of the structure of many Animals, especially of 

 the lower forms, of complex organs such as the vertebrate brain, 

 of embryology, and of the minute structure of cells and tissues 

 has been completely revolutionised. Specially remarkable is the 

 advance in our knowledge of the Sponges, Actinozoa, Echinoderms, 

 and Amphioxus. The discovery of the unsuspected chordate 

 affinities of Balanoglossus, Rhabdopleura, and Cephalodiscus is also 

 worthy of special mention. Probably the greatest of comparatively 

 recent embryological triumphs, belonging to the earlier part of the 

 period now under discussion, is Kowalewsky's discovery of the 

 notochord and hollow nervous system of the Tunicate larva, which 

 resulted in the removal of the Urochorda from Molluscoida to 

 Chordata, and in breaking down the sharp line between Vertebrates 

 and Invertebrates. 



But perhaps the most remarkable result of improved micro- 

 scopical technique is the rise and development of a distinct 

 department of histology, known as cytology, dealing with the 

 minute structure of the protoplasm and nuclei and the various 

 intra-cellular phenomena such as mitosis. Our knowledge of this 

 subject is entirely a product of the last twenty years, and is due in 

 great measure to the researches of W.Plemming, E. Strasburger, 



