LAW OF DEVELOPMENT KNOWN AS VON BAER's LAW. 75 



On the Law of Development commonly known 

 as von Baer's Law ; and on the Significance 

 of Ancestral Rudiments in Embryonic De- 

 velopment. 



By 

 Adam SedgAvick, M.A., F.R.S. 



The generalisation commonly referred to as v. Baer's law 

 is usually stated as follows :— Embryos of different members 

 of the same group are more alike than the adults, and the 

 resemblances are greater the younger the embryos examined. 

 It appears to be pretty clear that v. Baer held some such view 

 as this,' and there can be no doubt that it is a view which is 

 largely taught at the present day.^ In fact, I think it is safe 

 to say that all zoologists are brought up with this view as one 

 of the fundamental postulates of their science. 



It will be the object of the following pages to show that this 

 view is not in accordance with the facts of development. 

 V. Baer based his view mainly upon the study of Vertebrate 

 embryos; and it will be convenient for us, in criticising him, 

 to begin with an examination of this group. We may also, 

 in the first instance, follow v. Baer in another point, viz. in 

 limiting ourselves to the consideration of embryos as opposed 

 to larvae. Embryonic development and larval development 

 take place under entirely different conditions, and in order to 

 obtain clear ideas they must be considered apart in treating 



^ Vide 'Ueber Entwickelungsgesoh. d. Thiere,' part i, pp. 321, 223, 

 and 324. 



2 Vide Darwin in 'Origin of Species,' p. 364, 6tli ed. ; and Balfour, 

 ' Comp. Embryology,' vol. i, p. 2. 



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