THE HISTORY OF ZOOLOGY 



507 



young embryos are composed. In 1846 it was first applied to 

 the contents of the cell by von Mohl. Later it was discove 

 that there were cells without walls and that the protopla 

 was the real essential. The cell theory formed the foundation 

 of the science of histology and revolutionized the conceptions of 

 pathology. 



Ever since the spermatozoa had been discovered in [I 

 there had been much speculation as to their nature and func- 

 tion. Many held, as we have already noted, that they con- 

 tained a miniature model of the adult; others believed them to 

 be merely parasitic animals in the seminal fluid. It was nearly 

 the middle of the nineteenth century before the union of ovum 

 and spermatozoon was observed, and a few years later the 

 development of the spermatozoa from the cells of the testis 

 was ascertained; thus the part the}' play in fertilization was at 

 length determined, and the dispute between ovists and animal- 

 culists settled. 



The number of naturalists who made valuable and extensive 

 contributions to knowledge in zoology during the first half of the 

 century was very great, but in a brief review of the histon 

 the subject we must pass them over ; it will suffice to note a few 

 names, while many others are of almost equal importance. 

 Thus the Russian, von Baer (1792-1876), discovered in 1S2- the 

 mammalian ovum, and in his work on the chick confirmed 

 Wolff's account of the development of the organs from layers of 

 cells, the germ lavers. The English scientist, Owen ( 1 803- 1 8* 

 made very extensive studies of vertebrate anatomv, and wrote a 

 text-book on the subject. The French zoologist, the elder 

 Milne-Edwards (1800-1885), made valuable contributions to anat- 

 omy and phvsiology in his " Lecons de l'Anatomie et de la 

 Physiologie comparee." One of the most interesting figures of 

 the middle of the century was the celebrated Swiss naturalist, 

 Louis Agassiz( 1 807-1 873), who came to America, and as a pro 

 fessor at Harvard University founded the large school of natu- 

 ralists in the United States. He was the last of the great 

 scientists to hold to the fixity of species, and expressed his belief 

 in an essay, published by a curious coincidence in 1859, the year 

 in which Darwin's "Origin of Species" appeared. The wide influ- 

 ence of Cuvier in opposing the theory of evolution lends an 



