524 ZOOLOGY 



Italian physician, showed experimentally that maggots arose 

 from the eggs of flies. Harvey, to whom we have already 

 referred, and Malpighi (1628-1694) an Italian histologist and 

 one of the first to make much use of the microscope, were the 

 pioneers of embryology. They studied chiefly the development 

 of the chick and a few mammals. Harvey announced that all 

 animals arose from eggs. Malpighi thought that parts of the 

 embryo already existed in the egg in miniature. During the 

 next hundred years this theory (known 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 preforma- 

 tion theory, and held the egg to be unorganized at first, with no 

 trace of the future organs in it. He believed some vital prin- 

 ciple organized the undifferentiated material into an embryo. 

 Bitterly opposed at first, his general view came to dominate 

 Embryology. 



Von Baer (i 792-1 876) is recognized as the greatest of early 

 embryologists. He adopted the comparative point of view 

 in embryology, as Cuvier had for anatomy and Miiller 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) 



