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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



the adult state as is not inconsistent with the 

 supposition that the vertebrates and the ascidia 

 may have had a common ancestral form. Kowa- 

 levsky'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 ani- 

 mals, if we do not at once adopt the hypothesis 

 that through the ascidian and other forms the 

 origin of the vertebrates may be traced down- 

 ward in the series to the lower grades of animal 

 organization. 



The notochord extends a short way forward 

 into the cranial basis, and an interesting question 

 here presents itself, beginning with the specula- 

 tions 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 

 vertebrae. 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 homologous with verte- 

 bral metameres. 1 



The history of the formation of the vertebral 

 column presents an interesting example of the 

 correspondence in the development of the indi- 

 vidual and the race, in that all the stages which 

 have been referred to, as occurring in the grad- 

 ual evolution of the vertebral column in the series 

 of vertebrates, are repeated in the successive 

 stages of the embryonic development of the high- 

 er members of the scries. 



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 

 embryological terms, be styled the subcranial 

 plates. 



Without attempting to follow out the remark- 

 able changes which occur in the development of 

 the nose and mouth in connection with the an- 

 terior set of these plates, which, from being 



1 See the interesting and valuable memoirs of W. K. 

 Parker, ''On the Anatomy and Development of the Verte- 

 brate Skull." in "Transactions" of Royal Society, the' re- 

 searches of Gegenbaur, Mihalkovics, and more particularly 

 the memoir by F. M. Balfour, "On the Development of 

 the Elasmobranchs," in the Journal of Anatomy and 

 Physiology, vols. x. and xi. 



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 182G, and formed 

 the subject of an elaborate investigation by Rei- 

 chert in 1837. 



These plates consist of a series of symmetri- 

 cal 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, while in reptiles, birds, and 

 mammals, they undergo various changes leading 

 to the formation of very different parts, which 

 could not be recognized as having any relation 

 to gill-structure but for the observation of their 

 earlier embryonic condition. The history of this 

 part of development 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 trans- 

 formations of the first pair of branchial apertures, 

 which lead to the formation of the passage 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 cartilage of Meckel, 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. 



The other arches undergo transformations 

 which are hardly less marvelous, and the whole 

 scries of changes is such as never fails to impress 

 the embryological inquirer with a forcible idea of 

 the persistence of type and the inexhaustible va- 

 riety of changes to which simple and fundamental 

 parts may be subject in the process of their de- 

 velopment. 



It is also of deep significance, in connection 

 with the foregoing phenomena, to observe the 

 increase in the number of the gill-bars and aper- 

 tures as we descend in the scale to the cartilagi- 

 nous 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 ar- 



