514 ZOOLOGY 



as the preformation theory) took strong hold on the imagination 

 of the students of the subject, and the physiologists Haller, 

 Bonnet, Leibnitz, etc., developed the idea that the miniature 

 embryo preformed in the egg must in its turn contain the next 

 generation also preformed in miniature, and this the next, and 

 so on indefinitely. About 1677 it was discovered that the sperm 

 united with the egg, and some believed that the miniatures were 

 in the sperm rather than in the egg. Kaspar Wolff in 1759 

 attacked the whole preformation theory, and held the egg to 

 be unorganized at first, with no trace of the future organs in it. 

 He believed some vital principle organized the undifferentiated 

 material into an embryo. Bitterly opposed at first, his general 

 view came to dominate Embryology. 



Von Baer (1792-1876) is recognized as the greatest of 

 embryologists. He adopted the comparative point of view 

 in embryology, as Cuvier had for anatomy and Muller had for 

 physiology and with equal fruitfulness. Von Baer discovered 

 that all the higher types of animals develop first germ layers 

 (ectoderm, entoderm, and mesoderm), and that the organs are 

 formed by the growth and foldings of these. 



The increasing knowledge of cell structure and behavior 

 added greatly to progress in embryology. During these 

 years it became clear that the egg and the sperm were single cells, 

 that the fertilized ovum divides (cleavage) as ordinary cells do; 

 that the descendants of these cells become different in the 

 different layers, tissues, and organs, as they are formed. Gegen- 

 bauer, Koelliker, Huxley, Haeckel and many others made 

 contributions at these points. 



Building on the preceding work Francis Balfour (1851-1882) 

 of Cambridge, England, published in 1881 his monumental 

 "Comparative Embryology." It made available the best 

 conclusions of the past and developed general points of view 

 that had only slightly or not at all appeared. Many great 

 zoologists have given large attention to embryology since this 

 time. In Germany, Fol, Oscar Hertwig, William His, Roux; 

 and in America, Brooks, Minot, Whitman, Wilson, Loeb, and 

 Morgan have made brilliant contributions to our knowledge. 

 These include the study of the processes of fertilization and the 



