208 PROFESSOR MARSHALL AND EDWARD J. BLES. 



strict recapitulation. It is inconceivable that an ancestor of frogs 

 should have been an animal with a brain, a notochord, and a heart 

 with the characteristic Vertebrate twist, and with the main sub- 

 divisions already indicated, and yet that this heart should have had 

 no connection whatever with the branchial vessels or the aorta. 



We are as yet completely in the dark as to the steps by which the 

 branchial respiration, and consequently the branchial circulation, 

 of Vertebrates were first acquired. The development of the 

 branchial vessels in fishes has been as yet very imperfectly studied, 

 and we shall probably be wise in exercising great caution in 

 attempting to reconstruct the past ancestral history from the 

 development of these vessels in higher Vertebrates. It is commonly 

 assumed that the condition in which there is in each arch a con- 

 tinuous vessel connecting the heart and aorta directly is an ancestral 

 one ; and if this be the case — which is by no means proved — then 

 the mode of development described by Maurer in Puma esculenta 

 would be more primitive than that observed by us in Eana tenvporaria ; 

 and the exceptional tadpole of R. temporaria which we have described 

 above, could then be explained as a case of reversion to the more 

 ancestral type. 



We have thought it best to introduce this somewhat lengthy 

 discussion in this place, because it arises directly from our observa- 

 tions on the earlier stages, and because the later stages are of little 

 interest except in connection with the questions we have just con- 

 sidered. We will now return to the description of these later stages, 

 commencing with the tadpole at the stage when it emerges from the 

 spawn and becomes free. 



III. THE CONDITION OF THE HEART AND BLOOD VESSELS 

 IN 5 mm. TADPOLES. 



1. External Characters. 



As compared with the earlier stage, the newly hatched tadpole 

 differs chiefly in the greater size of the tail, to which the increased 

 length of the animal is entirely due, and which now forms about one- 

 third of the entire length. The abdomen is rather less bulky, owing 

 to the absorption of yolk for food. The sucker is very prominent, 

 and larger than before. The olfactory pit has a rather narrower 



