DEVELOPMENT OF THE EMBRYO. 1007 



of "diverging appendages" (a, a), which are well seen in the ribs of Birds; 

 and these are considered by Prof. Owen to be the fundamental elements of the 

 bones of the " extremities" or " limbs," those of the anterior extremity being 

 the diverging appendages of the occipital vertebra ( 1011), and those of the 

 posterior extremity standing in the same relation to one of the sacral vertebrse. 1 

 The extremities themselves are developed, in all Vertebrata, as leaf like ele- 

 vations from the parietes of the trunk (Fig. 269, <?, q, r, r) ; those peculiarities 

 .of form by which they are adapted to specialities of function being determined 

 by subsequent processes of development. Thus in the Human foatus, the fingers 

 are at first united by the primitive blastema, as if webbed for swimming ; but 

 this, as Prof. Miiller justly remarks, is less to be regarded as an approximation 

 to the form of the extremity characteristic of aquatic animals, than as the primi- 

 tive and most general form of the hand, the individual parts of which subse- 

 quently become more completely isolated in such animals as require to use them 

 separately. 



1011. It is in the cranial segments that the vertebral elements undergo 

 their most remarkable transformations, the departure from the " archetype" 

 being more complete in Man than in any other animal ; so that it is only by 

 tracing them through their simplest to their most complicated forms and ar- 

 rangements, that the true nature of the latter can be elucidated. 3 The number 

 of the segments entering into the skull has been a subject of much discussion 

 among those who adopt the " vertebral theory" of its composition ; but Prof. 

 Owen agrees with Oken (the original propounder of the theory) in fixing the 

 number at four, which corresponds with that of the primary divisions succeeding 

 each other in a linear series, that are distinctly marked in the early development 

 of the Encephalon, namely (proceeding from behind forwards), the Epencephalon, 

 the Mesencephalon, the Prosencephalon, and the Rhinencephalon ; 3 and also 

 corresponding with the number of the nerves of special sense, the Auditory, 

 Gustative, Optic, and Olfactory, which issue from this part of the neural axis 

 with the same segmental regularity that the ordinary sensori-motor nerves do 

 elsewhere. 4 



1 The beautiful chain of reasoning by which this position is, in the Author's opinion, 

 irrefutably established, is contained in the works of Prof. Owen already referred to ; a 

 sketch of it, and of the whole " Vertebral Theory," will be found in the Author's " Princ. 

 of Phys., Gen. and Comp.," | 320 et seq., Am. Ed. 



2 See especially Prof. Owen's "Archetype Skeleton," and the sketch contained in the 

 Author's "Princ. of Phys., Gen. and Comp.," %% 320 *, 320 < I, Am. Ed. 



3 The Rhinencephalon (consisting of the Olfactive ganglia) is seldom distinctly marked 

 out in the early stage of development of the higher Vertebrata, but is obvious enough in 

 Fishes (Fig. 171, A). 



4 Under the guidance of the unerring light of Comparative Anatomy and Development, 

 the composition of the Cranial portion of the skull consisting of the bodies and neural arches 

 of the four cranial vertebrae has been determined by Prof. Owen as follows, each of the 

 " elements" enumerated being marked as distinct, by the separateness of its centre of ossi- 

 fication. 



TABLE I. 



Composition of the Neural Arches of the Cranial Vertebrae, in Man. 

 I. EPENCEPHALIC OR OCCIPITAL VERTEBRA. 



Centrum ; Basi-occipital portion of the Occipital bone. 



p T ) Coalesced into the lateral or condyloid portions of the Occipital 



rarapopnyse& ( bone> the parapopliyses being mar ked by the scabrous ridge 



> 



; J giving attachment to the rectus lateralis muscle. 

 Neural Spine; Proper Occipital bone. 



II. MESENCEPHALIC OR PARIETAL VERTEBRA. 



Centrum ; Basi-sphenoid, or body of the posteiior or spheno-temporal part of the 

 Sphenoid bone. 



