LAW OF DEVELOPMENT KNOWN AS VON BAER’S LAW. oo 
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 Sedgwick, 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 larve. 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 
1 Vide ‘Ueber Entwickelungsgesch. d. Thiere,’ part i, pp. 221, 223, 
and 224, 
2 Vide Darwin in ‘Origin of Species,’ p. 364, 6th ed.; and Balfour 
‘Comp. Embryology,’ vol. i, p. 2. 
