DEVELOPMENT OF THE FORMS OF ANIMAL LIFE. 



497 



rangement of the gills is exactly similar to that 

 of the amphioxus. 



The study of the comparative anatomy of the 

 heart and its mode of formation in the embryo 

 furnishes also most striking illustrations of the 

 relation between ontogenetic and phylogenetic 

 development in the vertebrates, and is not with- 

 out its applications to some of the invertebrate 

 groups of animals. 



I need only recall to your recollection the 

 completely double state of this organ in warm- 

 blooded animals, by which a regular alternation 

 of the systemic and pulmonary circulations is 

 secured, and the series of gradations through 

 the class of reptiles by which we arrive at the 

 undivided ventricle of the amphibian, and the 

 further transition in the latter animals by which 

 we come at last to the single heart of fishes ; and 

 to state that in the embryo of the higher animals 

 the changes by which the double heart is ulti- 

 mately developed out of an extremely simple tu- 

 bular form into which it is at first moulded from 

 the primitive-formative cells are, in the inverse 

 order, entirely analogous to those which I 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 con- 

 tractile 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 ventiicle, 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 con- 

 ditions of foetal life prevent the return of arteri- 

 alized blood to the left auricle. The growth of 

 another septum within the ventricular portion 

 gradually divides that cavity into two ventricles, 

 repeating somewhat in its progress the variations 

 observed in different reptiles, and attaining its 

 complete state in the crocodile and warm-blood- 

 ed animals. 



I must not attempt to pursue this interesting 

 subject further, but I cannot avoid making ref- 

 erence to the instructive view presented by the 

 embryological study of the nature of the malfor- 

 mations to which the heart is subject, which, as 

 in many other instances, are due to the persist- 

 ence 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 interesting series of changes by which the 



32 



aortic bulb, remaining single in fishes, and serv- 

 ing 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 remarkable 

 transformations of the vascular arches which 

 proceed from the aortic bulb along the several 

 branchial arches, and which, in the gills of fishes 

 and aquatic amphibia, undergo that minute sub- 

 division which belongs to the vascular distribution 

 of gills, but which in the higher non-branchiated 

 animals are the subject of very different and va- 

 rious changes in the partial obliteration of some, 

 and the enlargement of others, by which the per- 

 manent vessels are produced. 



These changes and transformations have for 

 many years been a subject of much interest to 

 comparative anatomists, and will continue to be 

 so, not only from their presenting to us one of 

 the most remarkable examples of conformity in 

 the plan of development and the type of perma- 

 nent or completed organization in the whole 

 series of vertebrated animal?, but also because of 

 the manifest dependence of the phenomena of 

 their development upon external influences and 

 atmospheric conditions which affect the respira- 

 tion, nutrition, and modes of life, of the animal. 



Nor is the correspondence to which I now 

 refer entirely limited to the vertebrata. For 

 here, again, through the amphioxus and the as- 

 cidia, we come to see how an affinity may be 

 traced between organs of circulation and respira- 

 tion which at first appear to belong to very dif- 

 ferent types. The heart of vertebrates is, as is 

 well known, an essentially concentrated form of 

 vascular development in the ventral aspect of the 

 body ; while the heart of the invertebrate, wheth- 

 er in the more concentrated form existing in the 

 articulata and muscula, or in a more subdivided 

 shape prevalent in the annelida, is most frequent- 

 ly dorsal ; yet the main aorta of the vertebrates 

 is also dorsal ; and it is "not impossible, through 

 the intermediate form of amphioxus, to under- 

 stand how the relation between the vertebrate 

 and the invertebrate type of the blood-vascular 

 system may be maintained. 



But I am warned by the lapse of time that I 

 must not attempt to pursue these illustrations 

 further. In the statement which I have made of 

 some of the more remarkable phenomena of 

 organic production — too long, I fear, for your 

 endurance, but much too brief to do justice to 

 the subject — it has been my object mainly to 



