ANATOMY OF VERTEBRATES. 



3 



6, 



Stages of developement of a vertebrate genu (Rabbit), cvn. 



symmetry, constitutes the chief developmental characteristic of 

 the vertebrate animal. 



The twofold symmetry is shown in the bone-segment, fig. 7 ; 

 also in the flesh-segment surrounding the skeletal one in fig 

 in which the mid point . 



marks the ' noto- 

 chord;' with the neu- 

 ral canal above, the 

 haemal canal below ; 

 both surrounded by 

 the two neural and 

 two hamial masses of muscles on each side. 



The lancelet, Branchiostoma, fig. 23, superinduces its distinc- 

 tive characters upon this stage. Aponeurotic septa accompany the 

 pairs of nerves and divide the longitudinal muscular masses into 

 segments. At the next rise segmentation is shown by the develop- 

 ment of cartilage, forming pairs of plates, fig. 5, v, 

 commonly corresponding with the pairs of nerves 

 sent off from the neural axis, and with the pairs 

 of vessels from the hremal axis. As these plates 

 ossify, ossification commonly also begins at cor- 

 responding points of the notochord, dividing it 

 into as many central parts as there are peri- 

 pheral plates or arches, and constituting skeletal 

 segments or l vertebra? ; ' according, or reducible 

 to, the type, fig. 7. 



§ 2. Structural characters. — The series of i vertebra?,' under 

 their several modifications, as the neui'al or hremal organs 6 



may predominate, constitute the vertebral column. The 

 neural axis consists of ' encephalon' or brain, and of 

 f myelon' or spinal chord. The organs of the five senses 

 — touch, taste, smell, hearing, and sight — are usually 

 present. The blood-discs, fig. 8, speedily acquire the red 

 colour which, by their number and minuteness, they 

 impart to the whole blood. The heart is a compact mus- 

 cular organ, of two or more cavities, propelling the blood, 

 through a closed system of arteries and veins, directly to 

 the breathing-organ, and, in most vertebrates, directly also to the 

 body. The breathing-organ communicates with the pharynx. The 

 alimentary canal has distinct receptive and expellent apertures, 

 usually at opposite ends of the trunk. The mouth is provided 

 with two jaws, placed one above or before the other, working 

 in the direction of the axis of the body. The muscles surround 



B 2 



Germ of a Rabbit (Barry) 



V'; 

 Section of a 

 ' myocomma ' 

 or flesh-seg- 

 ment; tail of a 

 Mackerel.xxi. 



