43 8 NINETEENTH CENTURY. PT. IIL 



know that if you take a bird's egg when it is newly laid, yon 

 will see inside it a yellow yolk floating in a white fluid. 

 But if you take the egg after the mother-bird has sat upon 

 it for some days, the yolk will begin to have the form of a 

 bird, and if you were to go on taking an egg every day from 

 under a sitting hen each one would be found, when opened, 

 to be more like a chicken than the. one before it, until the 

 last, if you opened it just about the time when it ought to 

 be hatched, would be a perfect chicken, only that its feathers 

 would not be yet grown. Now the study of the different 

 stages of the development of the chicken in the egg, and of 

 all living beings going through the same stages, is called 

 Embryology, and has become of immense importance in the 

 history of animals. 



You will remember that Harvey, Malpighi, and many 

 other physiologists, occupied themselves with this study ; 

 but no discoveries of very great importance were made in 

 it before the time of Karl von Baer, a Russian anatomist, 

 who was born in 1792 and died in 1876. Von Baer was 

 the pupil of a very famous anatomist, Professor Dollinger, 

 and while he was working under him at Wiirzburg he made 

 for him a number of observations upon the growth of the 

 chicken in the egg, which led him to study the embry- 

 ology of animals, and to discover the remarkable law of 

 which we must now speak. 



Before Von Baer's time it had always been supposed that 

 the many kinds of animals, so different from each other, 

 must be quite unlike from the very first moment that they 

 began to grow, but Von Baer discovered that this is not so, 

 but that the embryos or beginnings of an ox, a bird, a lizard, 

 or a fish, are so like each other that they can only be dis- 

 tinguished by their size ; and, what is still more remarkable, 

 they remain alike till they have been growing for some time 



