XXI.] EMBRYOLOGY. 257 



air-breatliing Batrachians passes iu the course of its 

 ascending development.^ 



Another instance of the higher forms of a group passing Branchise 

 through stages of development similar to the mature °*^ *-"™^" 

 forms of the lower species of the same group, is afforded by 

 the branchiaB of the higher Crustacea, which in the course 

 of their development successively present the likeness of 

 the branchipe of different inferior orders of the same class. 2 



But probably the most instructive, if not the most obvi- 

 ously interesting, of all cases of development, are those of 

 the Vertebrata. I have spoken of the developmental 

 changes of the respiratory system in the Batrachians ; and 

 there are facts in the development of the circulatory system Circula- 

 of the air-breathing Vertebrata which, we can scarcely ^^°^ i". 

 doubt, are profoundly connected with these. It is obvious, embryo, 

 from the relation of respiration to the blood, that the posi- 

 tion of the respiratory organs, and of at least a portion of 

 the blood-vessels, must be determined the one by the 

 other. The blood must flow where it will be aerated : 

 consequently, the plan of so much of the circulation as 

 ministers to respiration must of necessity be quite different 

 in the air-breathing Vertebrata from what it is in fishes. 

 But in the embryos of the air-breathing Vertebrata, the 

 blood-vessels are at first formed on the same plan as in 

 fishes : the blood flows towards that part of the body 

 where the fish's branchiae are, and flows through arteries 

 which divide and reunite as the fish's branchial arteries 

 do ; wliile slits are formed between the arterial branches, 

 like those which in many fishes admit the water into the 

 gills, although no branchiae are ever formed. At a later 

 period of development the plan of the circulation, so far 

 as it is connected with the respiratory organs, is totally 

 changed, and the " branchial slits " on each side of the 

 neck close up and disappear.^ Another closely-related fact 

 is, that bodies (the " Corpora Wolffiana ") are formed in the 



1 Carpenter's Comparative Physiology, p. 706. 



^ Milne-Edwards, quoted in Carpenter's Comparative Physiology, 

 p. 745. 



^ Carpenter's Human Physiology, p. 7P9. 



S 



