GENEOLOGICAL. 369 



history of the chick in more exact detail. In 1824, 

 Prevost and Dumas observed the division of the 

 frog's ovum into masses. In 1827, Von Baer ful- 

 filled, after a century and a half, what De Graaf had 

 attempted, he discovered the mammalian ovum 

 and traced it from uterus to oviduct, and thence to 

 its position in the ovary itself. Soon afterwards, 

 Wagner, Von Siebold, and others elucidated what 

 was still hidden from Von Baer the real nature of 

 the spermatozoon. Kolliker began to trace the cells 

 into which the ovum divides to their results in the 

 tissues of the developing organism. In short, em- 

 bryology began to get a firm basis. 



Von 'Baer. The foundation of modern embry- 

 ology may be dated from the work of Karl Ernst von 

 Baer ( 1792-18 76). He broadened embryology as 

 Cuvier has broadened anatomy, as Johannes Miiller 

 afterwards broadened physiology, by making it 

 comparative. He showed how the development of 

 an embryo proceeded from the general to the spe- 

 cial. He was the first to show, though his own 

 illustrations have not survived, how embryological 

 facts may be of service in classification. 



Von Baer is linked to Francis Balfour by many 

 illustrious workers in embryology: Alex. Agassiz, 

 Claus, Gegenbaur, Goethe, Haeckel, His, Kolliker, 

 Kowalevsky, Leuckart, Loven, Metschnikoff, Jo- 

 hannes Miiller, Ratke, Remak, Sars, Semper, Van 

 Beneden, and many others. A strong stimulus was 

 given by Balfour's monumental text-book (1880- 

 1881), and in the last twenty years embryology has 

 been the most progressive department of biology. 



The Germ-Cells. The cell-theory (1838-39) 

 enunciated the important fnot that every mnlticellu- 

 lar organism, if reproduced in the ordinary way, be- 



