BILATERAL FORM OF VERTEBRATES. 257 



Fig. 54. — Transverse section of the same in the anterior part (through 

 the gill-body, at Teg, Fig. 53). 



Fig. 55. — Transverse section of the same in the central part (in the 

 neighbourhood of the heart, at hz, Fig. 53). 



Fig. 56. — Transverse section of the same in the posterior part (through 

 the ovary, at e, Fig. 53). The letters indicate the same parts in all the 

 sections. 



same relative position and connection ; but their positions 

 in relation to the central plane of section are exactly re- 

 versed ; the left half resembles the right, as though reflected 

 in a mirror. The two halves are called counterparts, or 

 antimera. The perpendicular line of section which divides 

 the two halves, passes from the back to the abdomen, and is 

 called the sagittal or dorso- ventral axis. If, on the other 

 hand, we make a horizontal section lengthwise through the 

 chord, the whole body falls into a dorsal and a ventral half. 

 The line of section which passes through the bod} T from the 

 right to the left side is called the cross or lateral axis. (Cf. 

 PJates IV. and V. 84 ) 



The two halves of the Vertebrate body which are 

 separated by this horizontal, transverse axis, have an 

 entirely different significance. The dorsal half is especially 

 the animal part of the body, and contains the greater part 

 of the so-called animal organs, of the nerve-system, muscle- 

 system, bone-system, etc. The ventral half, on the other 

 hand, is essentially the vegetative part of the body, and 

 contains the greater part of the vegetative organs of the 

 vertebrate, the digestive system, the reproductive system, 

 etc. The two outer secondary germ-layers are, therefore, 

 specially employed in the formation of the dorsal half, and 

 the two inner in the formation of the ventral half. Each 

 of the two halves develops in the form of a tube, and 

 surrounds a cavity in which another tube is enclosed 



