THE REPRODUCTION OF ANIMALS 237 



ment. By far the greater portion is occupied with the formation of the 

 tissues, organs, and systems of the finished embryo from the derivatives 

 and combinations of these primary tissues. In general, the outermost 

 layers of the body, the nervous system, and the sensory parts of the sense 

 organs are derived from ectoderm, the lining of the alimentary canal and 

 its derivatives — liver, pancreas, lungs, etc. — are derived from entoderm ; 

 the contractile and sustentative portions of the skin and the digestive 

 tract, the muscular, skeletal, circulatory, and excretory systems, and sus- 

 tentative and contractile tissues as a group are developed from mesoderm. 



Some Special Vertebrate Structures 



Somite formation. One of the fundamental devices of all vertebrate 

 organization, the linear repetition of such structures as the vertebrae, 

 ribs, spinal nerves, and certain blood vessels and thoracic muscles, is 

 foreshadowed and determined by the early development of mesodermal 

 somites. These are a linear series of paired, similar blocks that are formed 

 as centers of condensation and more rapid growth in that part of the 

 mesoderm that lies next to and on either side of the middorsal line of the 

 enibryo. The first pair of such centers forms on either side of what is to 

 be the midbrain of the embryo, and the formation of additional centers 

 proceeds rapidly backward (and slowly forward) from this point, each 

 center contiguous to the one in front of it. This somite formation is so 

 characteristic and regular that the number of somites that are visible 

 at any given time gives one of the best indications of the stage in develop- 

 ment reached by the early embryo, and much of the organization of the 

 future body parts is a consequence of somite formation. 



The notochord. In Amphioxus and in all vertebrate embryos the 

 formation of a characteristic axial rod, the notochord, takes place at 

 the same time as the formation of the mesoderm. In Amphioxus this 

 rod is derived from the middorsal line of the entoderm by an evagination 

 like that which produced the mesodermal pouches. The notochord, 

 however, does not show any metameric arrangement into a linear series 

 of pouches, nor does it develop a central cavity. It lies, as a cylindrical 

 axial rod, dorsal to the alimentary canal. In the vertebrates (but not in 

 Amphioxus) it is later replaced by the mesodermal tissues that form the 

 centra of the vertebrae. 



The hollow dorsal neural tube. Shortly after the beginning of the 

 formation of the mesoderm and notochord, the development of the nerv- 

 ous system begins. The ectoderm along the middorsal line begins to grow 

 rapidly, and a neural plate thicker than the rest of the ectoderm is formed. 

 This development is most rapid in the region of what is to become the 

 midbrain and progresses forward and backward from this point. The 

 plate soon sags below the level of the remainder of the ectoderm and 



