UNITY OF DKSIGN. o07 



served to pass, iu tlie embryonic stage of their 

 existence, through forms of transition, which suc- 

 cessively resemble these inferior classes. The 

 brain presents, in its earliest formation, a series of 

 tubercles, placed longitudinally, like those of fishes, 

 and only assuming its proper character at a later 

 period. The respiratory organs are at first branchiae, 

 placed, like those of fishes, in the neck, where 

 there are also found branchial apertures similar to 

 those of the lamprey and the shark ; and the heart 

 and great vessels are constructed like those of the 

 tadpole, with reference to a branchial circulation. 

 In their conversion to the purposes of aerial respi- 

 ration, they undergo a series of changes precisely 

 analo^•ous to those of the tadpole. 



Mammalia, during the early periods of their 

 developement, are subjected to all the transfor- 

 mations which have been now described ; com- 

 mencing with an organization corresponding to 

 that of the aquatic tribes; exhibiting not only 

 branchiae, supported on branchial arches, but also 

 branchial apertures in the neck; and thence pass- 

 ing quickly to the conditions of structure adapted 

 to a terrestrial existence. The developement of 

 various parts of the system, more especially of the 

 brain, the ear, the mouth, and the extremities, is 

 carried still farther than in birds. Nor is the 

 human embryo exempt from the same metamor- 

 phoses ; possessing at one period branchiae and 

 branchial apertures similar to those of the carti- 

 la<?inous fishes,* a heart with a single set of cavities, 



*■ These facts are given on the authorities of Rathke, Baer, 

 Huschke, Breschet, &c. Ann. des Sc. Naturelles, xv. 266. See 

 also the paper of Dr. A. Thomson, already quoted. 



