742 DEVELOPMENT OF THE LIMBS. 



In the higher animals the pneumo-gastric nerve shows no close 

 relation to the clefts, but in branchiate vertebrates it is continued past 

 the gills, and sends forked branches to the gill arches in front and 

 behind each of the clefts. 



3. ORIGIN AND FORMATION OF THE LIMBS. 



The close connection of the limb-arches with certain vertebral seg- 

 ments of the trunk has been previously referred to in the morpholo- 

 o-ical remarks, given under the description of the bones and muscles 

 m the first volume ; and although the vertebral homology of the parts 

 of the limb proper is not so apparent, at least in the proximal seg- 

 ments, yet in the quinquifid division of the more remote parts, in the 

 preaxial and postaxial arrangement of these divisions, and in their 

 relation to the nerves and some other circumstances, we can scarcely fail to 

 perceive some very near relationship between the structure of the limb 

 as a whole, and a certain number of the vertebral segments of the trunk. 



Fi£,'. 541. 



Fig. 541. — HujiAN Embryo of about 



FOUR WEEKS (froiu Kijllikei', after A. 



Thomson). | 



/, the anterior limb rising as a semi- 

 circular plate from the lateral ridge. 

 (The figure is elsewhere described. ) 



The limbs do not exist from 

 the earliest time of the forma- 

 tion of the cranio-vertebral part 

 of the trunk, but only begin to 

 be formed when the development 

 of the axial part of the body has 

 made some advance, as in the first half of tlie fourth day of incubation 

 in the chick, and at the commencement of the fourth week m the 



human embryo. . „ -, t r> ,, • -■ 



They first make their appearance as two pan-s of buds from the side 

 of the vertebral part of the trunk, in the form of flattish lateral elevations 

 with curved free margins projecting from the exterior of the body, 

 outside the thickened ridge (sometimes called the Wolffian ridgc) where 

 the division of the mesoblast into somatopleure and splanchnopleure 

 take place, and near the outer margins of the muscular plates. The 

 anterior pair of limbs appears earlier than the posterior, and for a long 

 time is always more advanced in the development of its parts. 



The place of formation of the auterior and posterior limbs does not 

 Tary to any great extent throughout the vertebrate animals,— and this 

 fact may be looked upon as one of the most marked features ot 

 vertebrate organisation. . ^■ ^ x. •:. • 



The thickened plate which forms the commencing limb, by its in- 

 creased growth, projects still more from the side, so as to take the 

 form of a flattened lappet with a semicircular free margin ; presenting 

 then two surfaces which may be named dorsal and ventral with reter- 

 ence to their correspondence to the like surfaces of the trunk, constituting 

 respectively the primitive extensor and flexor surfaces of the limb ; 

 while the anterior margin of the semicircular lappet corresponds to 

 the preaxial and the posterior margin to the postaxial borders of the 

 future limbs. 



