28 



Anatomy of the Rabbit. 



Branchiomerism is an adult feature of lower aquatic vertebrates, such 

 as fishes, where it appears as a succession of true, gill (or branchial) 

 structures, which support gill filaments as functional respiratory organs. 

 In higher terrestrial animals it appears as an embryonic feature (Fig. 19, 

 m.h. 1, 2) and is to be considered both as a determinant of adult form and 

 as a mark of aquatic ancestry. As in lower vertebrates, it underlies not 

 only structures of branchial significance (branchial arches in the restricted 

 sense) (Fig. 19, 1, 2), but also modified branchial structures, including 

 the first or mandibular arch (m.), and the second or hyoid arch (n.). So 

 great is the modification of these structures in passing from the embryonic 



eet. 



8 %-"% 





Fig. 20. Transverse section of a rabbit-embryo of about 10-£ days, 

 showing the arrangement of the organ-systems: ao., aorta; ch., not- 

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

 and sclerotomic divisions); e., primitive alimentary canal (enteron) ; 

 ect., ectoderm ; l.b., limb-bud ; ms., mesenchyme ; my,, external 

 portion of a myotome; n., nephrotome of embryonic kidney; inter- 

 mediate mass of mesoderm; sp. and so., splanchnic (visceral) and 

 somatic (parietal) portions of the ventral mesoderm. 



to the adult condition that the recognition of the ground-plan is perhaps 

 here of less general importance. It, however, 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 tenth cranial nerves; also the chief arterial vessels of the heart, 

 which are more fully referred to below. 



