CREATION BY EVOLUTION 



resemblances are very striking. For example, in groups as 

 distinct as flat-worms, annelids, and moUusks corresponding 

 cleavage cells give rise to similar organs, and the larvae of 

 these forms are very much alike, though the adult forms 

 are very different. 



In all vertebrates, from Amphioxus to man, there are many 

 fundamental resemblances which distinguish this one phylum 

 from all others. The eggs of vertebrates vary greatly in 

 size, ranging from the microscopic eggs of mammals to the 

 enormous ones of birds, and they differ also in the condi- 

 tions under which they develop. Some are set free as naked 

 cells to begin their lives independent of their parents; others 

 are enclosed in protective membranes and shells and are fur- 

 ther guarded and incubated by the mother or father; and 

 still others undergo their development within the body of 

 the mother. Associated with these varying conditions of 

 development are many differences in the size of eggs, the 

 rate of development, the methods of nutrition and other 

 features. But the striking fact remains that in all vertebrates, 

 from the lowest to the highest, one finds fundamental agree- 

 ment in the position and structure of the principal organs of 

 the body, and these organs arise from corresponding parts 

 of the egg or embryo in essentially the same manner in all. 

 Thus the brain and the nervous system come from a plate of 

 superficial cells on the dorsal surface of the gastrula — a plate 

 that rolls up at the sides to form a groove and then a tube. 

 The front end of this tube enlarges to form the brain; the 

 hinder part forms the spinal cord. The backbone, which gives 

 the name to the phylum (vertebrate or chordate) appears 

 as a row of cells, the notochord, above the alimentary canal 

 and below the nerve plate. This row of cells is the basis 

 of the backbone; it becomes surrounded by cartilage, is then 

 changed to bone and is segmented into the vertebrae of the 



[66] 



