ADDRESS. XCV 



to observe tho increase iu the number of the gill-bars and apertures as we 

 descend in the scale to tho cartilaginous fishes and lampreys, and the still 

 further multiplication of these metamoros or repeated parts in the Amphioxus ; 

 and it is interesting to note that in the Ascidia tho arrangement of the gills is 

 exactly similar to that of the Amphioxus. 



The study of the comparative anatomy of the heart and its mode of for- 

 mation in the embryo furnishes another striking illustration of the relation 

 between ontogenetic and phylogenetic development in the Vertebrates, and is 

 not without its applications to some of the invertebrate groups of animals. 



I need only recall to your recollection the completely doublo state of this 

 organ in warm-blooded animals, by which a regular alternation of the 

 systemic and pulmonary circulations is secured, — the series of gradations 

 through the class of Eeptiles by which we arrive at tho undivided ventricle 

 of the Amphibian, and the further transition in the latter animals by which 

 we come at last to the singlo heart of Fishes j and state that in the embryo 

 of the higher animals the changes by which the double heart is ultimately 

 developed out of an extremely simple tubular shape, into which it is at first 

 moulded from the primitive formative cells, are, in the inverse order, entirely 

 analogous to those which T have just now indicated as traceable in the 

 descending series of vertebrate animals ; so that at first the embryonic heart 

 of man and other warm-blooded animals is nothing more than a rhythmically 

 contractile vascular tube. By the inflection of this tube, the constriction of 

 its wall at certain parts, and the dilatation at others, the three chambers are 

 formed which represent the single auricle, the single ventricle, and the aortic 

 bulb of the fish. By later changes a septum is formed to divide the auricles, 

 becoming completed in all the air-breathing animals, but remaining incom- 

 plete in the higher animals so long as the conditions of fcetal life prevent the 

 return of arterialized blood to the left auricle. The growth of another septum 

 within the ventricular portion gradually divides that cavity into two ven- 

 tricles, repeating somewhat in its progress the variations observed in different 

 reptiles, and attaining its complete state in the crocodile and warm-blooded 

 animals. 



I must not attempt to pursue this interesting subject further; but I cannot 

 avoid making reference to the instructive view presented by the embryo- 

 logical study of the nature of the malformations to which the heart is sub- 

 ject, which, as in many other instances, are due to the persistence of 

 transitory conditions which belong to different stages of progress in the 

 development of the embryo. Nor can I do more than allude to the interest- 

 ing series of changes by which the aortic bulb, remaining single in fishes and 

 serving as the channel through which the whole stream of blood leaving the 

 heart is passed into the gills, becomes divided in the higher animals into the 

 roots of the two great vessels, the aorta and the pulmonary artery, and the 



