3IO 



NATURE 



\^A7tgust 1 6, 1877 



revolutionary in embryolopcal and zoological views, leading as 

 thev do to the support of the hypothesis that the ascidia is an 

 earlier stage in the phylogenetic history of the mammal and 

 other vertebrates. The analogy between the amphioxus and 

 ascidian larva is certainly most curious and striking as regards 

 the relation of the notochord to other parts, and it is not 

 difficult to conceive such a change in the form and position of 

 the organs in their passage from the embryonic to the adult 

 state as is not inconsistent with the supposition that the 

 vertebrates and the ascidia may have had a common ancestral 

 form. Kowalevsky's discovery opens up at least an entirely new 

 path of inquiry ; and we must be prepared to modify our views 

 as to the entire separation of the vertebrates from the other 

 groups of animals, if we do not at once adopt the hypothesis 

 that through the ascidian and other forms the origin of the 

 vertebrates may be traced downwards in the series to the 

 lower grades of animal organisation. 



The notochord extends a short way forward into the cranial 

 basis, and an interesting question here presents itself, beginning 

 with the speculations of Goethe and Oken, and still forming a 

 subject of discussion, whether the series of cranial or cephalic 

 bones is comparable to that of the vertebrce. On the whole it 

 appears to me that it is consistent with the most recent views of 

 the development and anatomy of the head to hold the opinion 

 that it is composed of parts which are to some extent homo- 

 logous with vertebral metameres.' 



The history of the formation of the vertebral column presents 

 an interesting example of the correspondence in the development 

 of the individual and the race, in that all the stages which have 

 been referred to, as occurring in the gradual evolution of the 

 vertebral column in the series of vertebrates, are repeated in the 

 successive stages of the embryonic development of the higher 

 members of the series. 



There is perhaps no part of the history of development in the 

 vertebrates which illustrates in a more striking manner the 

 similarity of plan which runs through the whole of them than 

 that connected with what I may loosely call the region of the 

 face and neck, including the apparatus of the jaws and gills. 

 The embryonic parts I now refer to consist of a series of 

 symmetrical pairs of plates which are developed at an early 

 period below the cranium, and may therefore, in stricter em- 

 bryological terms, be styled the subcranial plates. 



Without attempting to follow out the remarkable changes 

 which occur in the development of the nose and mouth in 

 connection with the anterior set of these plates, which, from 

 being placed before the mouth, are sometimes named preoral^ 

 I may here refer shortly to the history of the plates situated 

 behind the mouth, which were discovered by Rathke in 1826, 

 and formed the subject of an elaborate investigation by Reichert 

 in 1837. 



These plates consist of a series of symmetrical bars, four in 

 number in mammals and birds, placed immediately behind the 

 mouth, separated by clefts passing through the wall of the 

 throat, and each traversed by a division of the great artery from 

 the heart ; thus constituting the type of a branchial apparatus, 

 which in fishes and amphibia becomes converted into the well- 

 known gills of these animals, whilst in reptiles, birds and 

 mammals they undergo various changes leading to the formation 

 of very different parts, which could not be recognised as having 

 any relation to gill structure but for the observation of their 

 earlier embryonic condition. The history of this part of de- 

 velopment also possesses great interest on account of the 

 extraordinary degree of general resemblance which it gives to 

 the embryos of the most different animals at a certain stage of 

 advancement — so great, indeed, that it requires a practised eye 

 to distinguish between the embryos of very different orders of 

 mammals, and even between some of them and the embryos of 

 birds or reptiles, as well as in connection with the transforma- 

 tions of the first pair of branchial apertures, which lead to the 

 formation of the p.assage from the throat to the ear in the higher 

 vertebrata. There is equal interest attached to the history of the 

 development of the first pair of arches which include the basis of 

 formation of the lower jaw with the so-called cartilas:f 0/ Affciii, 

 and which, while furnishing the bone which suspends the lower 

 jaw in reptiles and birds, is converted in mammals into the 

 hammer-bone of the ear. 



^ See the interesting and valuable memoirs of W. K. Parker. "On the 

 Anatomy and Development of the Vertebrate Skull." in Trans, of Roy. 

 Snc. the researches of Gegenbaur, Mihalkovics, and more particularly the 

 Memoir by F. M, Balfour, " (In the Development of the Elasmobranchs," 

 in the Jfiim. 0/ Anat, atid Physiol., vols. x. and. xi. 



The other arches undergo transformations which are hardly 

 less marvellous, and the whole series of changes is such as never 

 fails to impress the embryological inquirer with a forcible idea 

 of the persistence of type and the inexhaustible variety of 

 changes to which simple and fundamental parts may be subject 

 in the jirocess of their development. 



It is also of deep significance in connection with the foregoing 

 phenomena, to observe the increase in the number of the gill- 

 bars and apertures as we descend in the scale to the cartilaginous 

 fishes and lampreys, and the still further multiplication of these 

 metameres or repeated parts in the amphioxus ; and it is, 

 perhaps, also interesting to note that in the ascidia the arrange- 

 ment 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 without its applica- 

 tions 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 ultimately developed out of an extremely simple 

 tubular 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 contractile v,ascular 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 incomplete in the higher animals so long 

 as the conditions of foetal life prevent the return of arterialised 

 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-blooded animals. 



I must not atteiTipt to pursue this interesting subject further, 

 but I cannot avoid making reference to the instructive view 

 presented by the embryological study of the nature of the 

 malformations to which the heart is subject, 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 threat vessels, 

 the aorta and the pulmonary artery, and the remarkable transfor- 

 mations 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 thatl minute subdivision 

 which belongs to the vascular distribution o gills, but which in 

 the higher non-branchiated animals are the subject of very 

 different and various changes in the partial obliteration of some, 

 and the enlargement of others, by which the permanent 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 develop- 

 ment and the type of permanent or completed organisation in the 

 whole series of vertebrated animals, but also because of the 

 manifest dependence of the phenomena of their development 

 upon external influences and atmospheric conditions which 

 affect the respiration, 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 ascidia, we come to see how an affinity may be traced 

 between organs of circulation and respiration which at first 

 appear to belong to very different types. The heart of verte- 

 brates is, as is well known, an essentially concentrated tonn of 



