xeiv report — 1877. 



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 embryological terms, be styled the subcranial plates. 



Without attempting to follow oirt the remarkable changes which occur in 

 the development of the nose and mouth in connexion 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 Eeichert 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 recognized as having any relation to gill-structure, but for the obser- 

 vation of their earlier embryonic condition. The history of this part of deve- 

 lopment also possesses great interest on account of the extraordinary degree 

 of general resemblance which it gives to the embryos of man and the most 

 different animals at a certain stage of advancement (so great, indeed, that it 

 requires a practised eye to distinguish between them though belonging to 

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

 of birds or reptiles), as well as in connexion with the transformations 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 cartilaye 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 marvel- 

 lous, 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 process of their development. 



It is also of deep significance, in connexion with the foregoing phenomena, 



