HISTORY OF ZOOLOGY 13 



during the third decade of the last century and three decades later was 

 completely remodelled by the protoplasm theorj' of Max Schultze. In 

 the cell theory a simple principle of organization was found for all living 

 creatures, and the wide rea m of histology was open for scientific treat- 

 ment. But the theory was of the greatest importance for developmental 

 physiology, for only with the recognition of egg, spermatozoon, and 

 cleavage spheres as nucleated cells, was there a sound basis for the solution 

 of the problems of fertilization, heredity, and embryonic differentiation 

 and for the experimental proof of hypotheses. 



With the establishment and systematic use of comparative anatomy, 

 the cell theory and histology, the ground was prepared for the series of 

 researches which characterized the second half of the nineteenth century. 

 Great advances were made in vertebrate anatomy by the classic researches 

 of Owen, Johannes MuUer, Rathke, Huxley, Gegenbaur and others; 

 our conceptions of organization have been completely altered by the work 

 of Dujardin, Max Schultze, Haeckel, and others, who have proved the 

 unicellularity of the lowest animals. The germ-layer theorjr was further 

 developed by Remak and KoUiker; and applied to the invertebrates by 

 Kowalewsky, Haeckel, and Huxley. It is beyond the limits of this brief 

 historical summary to go into what has been accomplished in other 

 branches of the animal kingdom; it must here suffice to mention the most 

 important changes which the Cuvierian system has undergone under the 

 influence of increasing knowledge. 



Changes in the System. — Of the four t3'pes of Cuvier the branch 

 Radiata was the one of which he had the least knowledge; it was 

 also the least natural, since it comprised, besides the radially sym- 

 metrical coelenteraites and echinoderms, other forms, which, like the 

 worms, were bilaterally symmetrical, or, like many infusorians, were 

 asymmetrical. C. Th. von Siebold introduced the first important change. 

 He limited the type Radiata, or, as he termed them, the Zoophytes, to 

 those animals with radially symmetrical structure (Echinoderms and the 

 Plant-animals) ; separating all the others, he formed of the unicellular 

 organisms the branch of 'primitive animals' or Protozoa; the higher 

 organized animals he grouped together as worms or Vermes; at the same 

 time he transferred a part of the Articulata, the annelids, to the worm 

 group, and proposed for the other articulates, crabs, millipedes, spiders, 

 and insects, the term Arthropoda. 



Leuckart, about the same time (1848), divided the Radiata into two 

 branches differing greatly in structure. The lower forms, in which no 

 separate body-cavity is present, the interior of the body consisting only 

 of a system of cavities serving for digestion, he called the Ccelenterata 



