STRUCTURE OF THE VERTEBRATES 21 



have tubes which conduct them to the outside, while the s^Derms 

 retain the primitive condition. 



Nervous System. Amphioxus has no brain, the anterior part 

 of the nerve cord being even smaller than that lying farther 

 posteriorly. This is considered a specialized condition, for the 

 tunicate has a better developed brain than the cephalochordates. 

 At the extreme anterior end of the nerve cord are two so-called 

 cranial nerves, and between them a light-sensitive, pigmented 

 eye spot. Posterior to this region are the metameric spinal 

 nerves. Each has two roots arising from the spinal cord, a 

 dorsal root and a ventral root. The dorsal is a mixed nerve, 

 being partly sensory and partly motor, while the ventral is 

 entirely motor. Differing from the vertebrate, all the nerve 

 cells are contained within the cord; and the roots do not unite 

 into a single spinal nerve, but each retains its independence 

 and ramifies into the tissues of the body. 



Embryology of Amphioxus. Amphioxus in its development is 

 properly considered the basis for all vertebrate embryology. Its 

 study has cleared many obscure points due to its perfectly gen- 

 eralized developmental stages, and the leisurely manner of organ 

 formation in the early embryo. 



The ova are transparent objects on the verge of human vision. 

 Both eggs and sperms are thrown into the water about dusk 

 during the breeding season, where fertilization takes place. The 

 ovum has a small amount of yolk material slightly concentrated 

 at the ventral pole. This is of more than passing significance 

 in cleavage, as it is a general rule that cells loaded with inert 

 material cleave more slowly than others. 



About an hour after fertilization the egg divides, the first 

 cleavage being through the dorso-ventral axis. The second is in 

 the same direction, at right angles to the first. The third 

 cleavage is horizontal, perpendicular to the first two, and 

 slightly asymmetrical, the four dorsal cells being slightly smaller 

 than the four ventral. Due to the use of cellular materials to 

 supply energy for cleavage, and the natural tendency of a 

 liquid to assume a spherical shape, a cavity has appeared in the 

 center of the eight-celled embryo, called the segmentation 

 cavity. 



