EMBRYOLOGY. 



The marvellous phenomenon of the evolution of a highly 

 complicated living being from a simple undifferentiated germ in 

 which it needs the aid of the most modern microscopical appli- 

 ances to detect any visible signs of life, has not unnaturally 

 attracted the attention of biologists from the very earliest periods. 

 Before the establishment of the cell theory the origin of the 

 organism from the germ was not known to be an occurrence 

 of the same nature as the growth of the fully formed individual, 

 and Embryological investigations were mixed up with irrelevant 

 speculations on the origin of life 1 . 



The difficulties of understanding the formation of the indivi- 

 dual from the structureless germ led anatomists at one time to 

 accept the view "according to which the embryo preexisted, 

 " even though invisible, in the ovum, and the changes which 

 " took place during incubation consisted not in a formation of 

 " parts, but in a growth, i.e. in an expansion with concomitant 

 " changes of the already existing germ." 



Great as is the interest attaching to the simple and isolated 

 life histories of individual organisms, this interest has been 

 increased tenfold by the generalizations of Mr Charles Darwin. 



It has long been recognized that the embryos and larvae 

 of the higher forms of each group pass, in the course of their 

 development, through a series of stages in which they more 

 or less completely resemble the lower forms of the group 2 . 

 This remarkable phenomenon receives its explanation on Mr 

 Darwin's theory of descent. There are, according to this theory, 

 two guiding, and in a certain sense antagonistic principles which 

 have rendered possible the present order of the organic world. 

 These are known as the laws of heredity and variation. The 

 first of these laws asserts that the characters of an organism 



1 To this general statement Wolff forms a remarkable exception, for though 

 without any clear knowledge of what we call cells he had very distinct notions on the 

 relations of growth and development. 



* Von Baer who is often stated to have established the above generalization really 

 maintained a somewhat different view. He held (Ueber Entwickelungsgeschichte d. 

 Thiere, p. 724) that the embryos of higher forms never resembled the adult stages of 

 lower forms but merely the embryos of such forms. Von Baer was mistaken in thus 

 absolutely limiting the generalization, but his statement is much more nearly true than 

 a definite statement of the exact similarity of the embryos of higher forms to the 

 adults of lower ones. 



