42 



ANA'l'()M^■ ov rill': RAHin r 



irsi)ira(()i-\' organs. In lii^licr Icnwsdial animals it appears as an 

 onibryonic fcalurr [Vlu;. 21 , in., h., 1, 2) and is to be eonsidered both 

 as a determinant of adnlt form and as a mark of aqnatic ancestry. 

 It mulcrlies (he arrani^ement not onl>- of structnres which in the 

 lower vertel)rates belong- to fnnctional gills (branchial arches in the 

 restricted sense) but also of modified branchial structures such as 

 the first visceral, or mandibular arcli (m.) and the second visceral, 

 or liyoid arch (h.). The modification of these structures in passing 



ect. 



V fns 



\^?':-:-- 



Fig. 22. Transverse section of a rabbit-embryo of about 10 V2 days, 

 showing the arrangement of the organ-systems: ao., aorta; ch., notochord; 

 coe., coelomic cavity; d.m., dorsal mesoderm (myotomic and sclcrotomic 

 divisions); e., primitive alimentary canal (cnteron); ect., ectoderm; l.b., 

 limb-bud; ms., mesenchyme; my., external portion of a myotome; n., neph- 

 rotome of embryonic kidney; intermediate mass of mesoderm; sp. and so., 

 splanchnic (\isceran and somatic (parietal) portions of the ventral mesoderm. 



from the embryonic to the adult condition is very great, but their 

 arrangement determines the position and relations of certain skeletal 

 structures, including the auditory ossicles, the hyoid, and in part 

 the laryngeal cartilages — a point of some value in the classification 

 of the parts of the head skeleton. It also determines the succession 

 of certain soft structures, including- the fifth, seventh, ninth, and 



