VON BAER'S LAW OP 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 foetal 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. THE 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 Eeproductive 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 reflect that, even 

 when he has attained the furthest limits of his Science, by- 

 advancing to those general principles which tend to place it 



