58 HUMAN BIOLOGY 



efficient type of locomotor apparatus, the starting-point, 

 as we shall see, of a long series of successive modifications, 

 from the lancelet to man. 



The developmental history of Amphioxus and other 

 lowly chordates (Delage) indicates that at a very early 

 period some two-layered, jellyfish-Iike forms gave up the 

 free-floating hfe of helpless plankton and began to wriggle 

 on the bottom of the rich inshore feeding-grounds. At this 

 time perhaps the mesodermic pouches already mentioned 

 as being on either side above the primitive gut became 

 rhythmically contractile, hke the bell of a jellyfish. While 

 the details are quite obscure, it is plain that some such 

 stage must have preceded the appearance of the perfected 

 muscle pouch, which is the unit of the locomotor apparatus 

 of all vertebrates and one of the most important of the 

 basic patents to which all vertebrates, including man, owe 

 the possibility of their subsequent careers. 



These primitive contractile pouches probably at first 

 surrounded the little bags of potential eggs or sperm which 

 had been derived from the walls of the primitive gut; even 

 in the embryos of higher vertebrates (including man) the 

 blocks of tissue which give rise to the body muscles and to 

 the segments of the backbone first appear on either side, 

 above the primitive gut and above the longitudinal strip 

 of tissue that gives rise to the eggs or sperm. Meanwhile at 

 a very early period the mesoderm began to develop a median 

 longitudinal groove. This groove, at first opening below into 

 the primitive gut, finally became closed off as a tube filled with 

 clear elastic tissue (Shumway). Why it did this we do not 

 know, but the step was of momentous consequence to the 

 future history of the race, for thereby the beginnings of a 

 backbone were attained. 



Along the middle of the back behind the brain and above 

 the notochord was a long paired groove or tube forming the 

 main nerve cord, with lateral branches, the spinal nerves, 

 leading out to the muscle pouches, which, as we have seen, 

 had already budded off from the primitive gut. By means of 

 these primitive "spinal nerves" and of the spinal cord and 

 brain, the contraction of the segmental muscle pouches 

 could be timed and integrated effectively into cooperating 



