INTRODUCTION 459 



organ systems conform to generalized, basic plans. After the generalized plan 

 of a particular system is established, it is modified in later development to fit 

 the requirements of the habitat in which the particular species lives. In the 

 cephalochordate, Amphioxus, a similar body form also develops, although it 

 is considerably modified. 



The common, generalized, primitive embryonic body form of all vertebrate 

 embryos possesses the following characteristics: 



( 1 ) It is an elongated structure, cyHndrical in shape, and somewhat com- 

 pressed laterally. 



(2) It is composed of five, basic, organ-forming tubes, oriented around a 

 primitive axis, the notochord (fig. 217). 



(3) It possesses the following regions: (a) head, (b) pharyngeal area, 

 (c) trunk, and (d) tail (figs. 217, 226, 227, 230, 238, 244, 246). 



In Chapter 1 1 and the following chapters, various details of these common 

 regions and other features will be considered. In this chapter, we are con- 

 cerned mainly with tubulation and antero-posterior extension of the major 

 organ-forming areas in relation to body-form development. 



3. Starting Point for Tubulation 



The starting point for tubulation of the major organ-forming areas and 

 subsequent, primitive, body formation is the gastrula; which, as observed in 

 Chapter 9, exists in two forms, namely, rounded and the flattened gastrulae 

 (figs. 219, 232). Many heavily yolked embryos, such as the embryo of Nec- 

 turus maculosus, although they form a rounded gastrula, are faced with some 

 of the problems of the flattened gastrulae (fig. 227). The rounded gastrulae, 

 found in the frog, Amphioxus, etc., differ from the flattened gastrulae present 

 in the bird, reptile, mammal, and teleost and elasmobranch fishes, mainly by 

 the fact that, at the beginning of tubulation and body formation, the epidermal 

 and gut areas already are partially tubulated in the rounded gastrulae. That is, 

 in the rounded blastoderm, the initial stages of tubulation occur in these two 

 major organ-forming areas during gastrulation. This means that the ventral 

 portion of the trunk area in rounded gastrulae is circumscribed by intact cellular 

 layers of the embryonic trunk region, with yolk material contained within the 

 cell layers, while, in flattened gastrulae, the ventro-lateral portions of the 

 trunk region are spread out flat, the yolk not being surrounded by the future, 

 ventro-lateral walls of the embryonic trunk region. These conditions are illus- 

 trated in figures 219B and C and 234A-F. 



The developmental problems faced by these two groups of gastrulae, there- 

 fore, are somewhat different. Moreover, tubulation of the organ-forming areas 

 and the development of body form in Amphioxus varies considerably from 

 that of the rounded gastrulae of the vertebrate group. For this reason, tubula- 

 tion in Amphioxus is considered separately. 



