YON BAER’S LAW OF DEVELOPMENT. 591 
characteristically-different structure in each of the classes of 
Vertebrata, which is not presented at any period in the history 
of the others. So, the evolution of the Circulating apparatus 
commences in all Vertebrata upon the same original plan ; and 
from this plan there is but little departure in the Fish ; but 
the circulating apparatus of the early Human embryo, how- 
ever like that of the adult Fish, differs from it in this essential 
particular,—the absence of gill-tufts receiving capillary 
vessels from the branchial arches (§ 286). The like is true 
in regard to the Nervous centres ; for although the earliest 
condition of the Human brain very closely resembles that of 
the brain of the fetal Fish, it never bears any exact analogy 
to that of the adult Fish. 
765. Hence the principal facts of Organic Development 
admit of being stated in this general formula, which we owe 
to the sagacity of Von Baer,—that the more special forms of 
structure arise progressively out of the more general,—a prin- 
ciple than which there is none more comprehensive or more 
important in the whole range of Physiological Science. 
766. Tue Unity of Plan which is visible through the whole 
Animal Kingdom, is nowhere more remarkable than in the 
function of which an outline has now been given. We have 
seen that, however apparently different, the essential character 
_ of the Reproductive process is the same in the highest Animal 
as in the lowest. It has been shown that the development of 
the highly-organized body of Man,—though it is to serve as the 
instrument of those exalted faculties, by the right employment 
of which he is made “ but a little lower than the Angels,”— 
commences from the same starting-point with that of the 
meanest creature living: for even Man, in all the pride of his 
philosophy, and all the splendour of his luxury, was once but a 
single cell, undistinguishable, by all human means of observa- 
tion, from that which constitutes the entire fabric of the 
simplest Protozoon. And when the Physiologist is inclined 
to dwell unduly upon his capacity for penetrating the secrets © 
of Nature, it may be salutary for him to refiect that,—even 
when he has attained the furthest limits of his Science, by 
advancing to those general principles which tend to place it 
