Generai, Plan of Organization. 



27 



organs of both sexes comprise the paired kidneys and ureters and the 

 unpaired urinary bladder. 



Certain organs of the body are not included in this classification: 

 (1) The thymus and thyreoid glands are connected with the digestive tube 

 in the embryonic condition, and for this reason are sometimes included 

 with the digestive system, although in the adult they occur as in- 

 dependent structures. (2) The suprarenal body is situated close to the 

 kidney of either side, but is independent of the latter, both in the adult 

 condition and in point of development. (3) The special (olfactory, optic 

 and auditory) sense-organs of the head are highly elaborated structures, 

 the relations of which are partly with the central nervous system. 



General Organization. — In the rabbit, as in all vertebrates, the 

 general plan of organization involves three chief features, as follows: 

 (1) axial orientation — the arrangement of the chief organ-systems 

 longitudinally about a more or less central, axial support; (2) meta- 

 merism — the arrangement of a con- 

 siderable portion of the body on a 

 segmented or metameric plan, 

 according to which structures are 

 repeated in a serial fashion along 

 the axis ; (3) branchiomerism — the 

 arrangement of a small anterior 

 portion of the body on a serial but 

 branchial plan, the latter depending 

 not on the succession of true 

 metameres but of visceral arches. 



The extent to which the general 

 disposition of the organ-systems is 

 dependent on a fundamental plan will 

 be evident from a comparison of the 

 accompanying figure (2U) of a trans- 

 verse section of the rabbit-embryo, or 

 of the schematic section of a general- 

 ized vertebrate (Fig. 21), with the 

 actual sections of the rabbit-foetus 

 as given in the plates, more especially 

 the abdominal section of Plate VIII and the thoracic section of Plate 

 VII. It will be considered more fully below in connection with the 

 general features of the organs. 



Metamerism (Fig. 19, me.) is characteristic of a dorsolateral portion 

 of the body, identifiable in the embryo as that including the dorsal and 

 intermediate portions of the middle layer or mesoderm (Fig. 2t), d.m. 

 and n.). In the adult it determines a number of features of serial 

 arrangement, including the succession of the vertebrae and ribs, the 

 divisions of the related dorsal musculature and its extensions to the ribs 

 and abdominal wall, and indirectly the succession of the spinal nerves 

 and their branches, of the parietal branches of the aorta, and the parietal 

 roots of the inferior caval vein, as well as of the tributaries of the azygos 

 vein. 



Fig. 19. Rabbit-embryo of 10^ days (4.8 

 mm.): m.. mandibular; h., hyoid; 1 and 2, first 

 and second branchial arches; a.l., anterior 

 limb-bud ; me., metameres; p.L, posterior 

 limb-bud. (After Minot and Taylor, in 

 Keibels Normentafeln, V.; Fig. 12.) 



