INTRODUCTION 455 



C. Notochordal area 



D. Lateral constrictive movements 



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 



b. Position occupied by the notochord and mesoderm at the end of gastrulation 



2. Neuralization and the closure of the blastopore 



3. Epidermal tubulation 



4. Tubulation of the entodermal area 



a. Segregation of the entoderm from the chordamesoderm and the formation of 

 the primitive metenteric tube 



b. Formation of the mouth, anus, and other specialized structures of the metenteron 



5. Tubulation of the mesoderm 



6. Later differentiation of the myotomic (dorsal) area of the somite 



7. Notochord 



F. Early development of the rudiments of vertebrate paired appendages 



G. The limb bud as an illustration of the field concept of development in relation to the 

 gastrula and the tubulated embryo 



H. Cephalic flexion and general body bending and rotation in vertebrate embryos 

 L Influences which play a part in tubulation and organization of body form 

 J. Basic similarity of body-form development in the vertebrate group of chordate animals 



A. Introduction 



1. Some of the Developmental Problems Faced by the 

 Embryo After Gastrulation 



a. Tubulation 



One of the main problems, confronting the embryo immediately following 

 gastrulation, is the tubulation of the major organ-forming areas, namely, 

 epidermal, neural, entodermal, and the two, laterally placed, mesodermal 

 areas. The epidermal, neural, and entodermal areas eventually form elon- 

 gated, rounded tubes, whereas the mesodermal tubes are flattened. The epi- 

 dermal and neural tubes extend the entire length of the developing embryo 

 (fig. 217A-C), while the entodermal tube normally terminates at the begin- 

 ning of the tail (fig. 217B, C), although in some instances it may extend even 

 to the tail's end (fig. 217A). Anteriorly, the entodermal tube ends along the 

 ventral aspect of the developing head (fig. 217A, C). The two mesodermal 

 tubulations are confined mainly to the trunk region of the embryo, but in the 

 early embryo of the shark they continue forward into the head almost to the 

 posterior limits of the developing eyes (fig. 217D). The condition of the 

 mesodermal tubes in the Amphibia resembles to a degree that in the shark 

 embryo (fig. 217B, E). 



An important concept to grasp is that the tubulations of the respective areas 

 occur synchronously or nearly so. It is true that the initial stages of the epi- 

 dermal and entodermal tubulations slightly precede the other tubulations in 



