494 DEVELOPMENT OF PRIMITIVE BODY FORM 



are effected at the level of the notochord and lower margin of the somitic 

 area from the anterior trunk region caudally into the tail. As a result, a 

 transverse section of the early vertebrate body appears pyriform or pear 

 shaped, with the neck of the pear directed dorsally (fig. 241C). The con- 

 striction line is shown typically in the developing embryo of Necturus (fig. 

 227) where it extends from the lower aspect of the head outgrowth along the 

 lower boundary of the somitic area to the base of the tail. A line, drawn 

 across the body from the general area of the two lateral constrictions and 

 passing through the notochord, divides the embryonic body into an upper or 

 epaxial (epiaxial) region above the level of the notochord and a lower or 

 hypaxial (hypoaxial) region below the level of the notochord. 



E. Tubulation of the Neural, Epidermal, Entodermal, and Mesodermal, 

 Organ-forming Areas in Amphioxus 



1. Comparison of the Problems of Tubulation in the Embryo 



OF Amphioxus with that of the Embryos in the 



SuBPHYLUM Vertebrata 



a. End-bud Growth 



In Amphioxus, the procedures involved in tubulation of the major organ- 

 forming areas and development of primitive body form differ from those in 

 the vertebrate group. For example, in the latter group, the basic rudiments of 

 the head, pharyngeal, trunk, and tail regions appear to be well established 

 at the end of gastrulation. During tubulation of the major organ-forming areas, 

 these subregions become extended in an antero-posterior direction and the 

 rudiments of specific structures begin to express themselves. This is especially 

 true of the head, pharyngeal, and trunk regions. The vertebrate tail, however, 

 arises from an end-bud tissue which progressively lays down the various parts 

 of the tail by means of a proliferative growth in the caudal direction. On the 

 other hand, in Amphioxus, only a small portion of the anterior end of 

 the future body is laid down during gastrulation. Further development of the 

 epidermal, neural, enteric, and mesodermal cellular areas together with the 

 notochord are dependent upon cell proliferation at the caudal end of the late 

 gastrula and later embryo. Much of the body of Amphioxus, therefore, is 

 formed by a caudal proliferative growth of end-bud cells, somewhat com- 

 parable to the end-bud growth of the tail in the vertebrate group. 



b. Position Occupied by the Notochord and Mesoderm at the End 



of Gastrulation 



A second feature of difference in the developing embryo of Amphioxus 

 from that of the vertebrate embryo lies in the arrangement of the notochord- 

 mesoderm complex of cells in the late gastrula. In the late gastrula of 



