44 THE EVOLUTION OF MAN 



mentary system; the mesoderm, the locomotor apparatus, and 

 the sex cells for the preservation of the race; and the ecto- 

 derm, the nervous system which is placed in control of the 

 body and puts man in touch with his environment. This 

 method of development is essentially similar in other animals. 



Up to this point the embryo appears as a rather simple 

 multicellular animal of the invertebrate type. The first indi- 

 cation that it is to become a vertebrate is the development of 

 a dorsal, longitudinal, rod-like axis, called the notochord, 

 which eventually extends posteriorly from the base of the 

 brain through the length of the body. In the lowest forms of 

 aquatic vertebrates this is the only longitudinal supporting 

 axis the body ever possesses, but in the higher fishes and the 

 terrestrial vertebrates, where a more stable axis is necessary, 

 the notochord is replaced by a more rigid, segmented, bony 

 structure, the vertebral column. 



The rudiment of the nervous system now appears, anterior 

 to the primitive streak and dorsal to the notochord, as a longi- 

 tudinal groove, known as the neural groove (Fig. 13), in the 

 dorsal surface of the thickened ectoderm. The lateral edges 

 of the groove become elevated and meet and fuse above it 

 in the mid-dorsal line and thus form a hollow, ectodermal tube, 

 called the neural tube. Practically the entire nervous system is 

 developed from the cellular walls of this structure. Shortly 

 afterwards a part of the yolk sac lying under the embryo is 

 folded off and this process results eventually in the formation 

 of an endodermal tube under the notochord which is the rudi- 

 ment of the alimentary canal (Fig. 12 D). 



Also the mesoderm, which at this stage is to be found lying 

 along each side of the neural canal, becomes cleft transversely, 

 beginning first just behind the brain, and this results in the 

 formation of a linear series of segments, or myotomes (Fig. 

 14), extending the length of the body of the embryo. This 

 primitive segmentation persists in a modified form in adult 



