DEFINITIONS AND CONCEPTS 391 



pattern of the gastrula results in the production of a two-layered form, for 

 example, as in Amphioxus; in others (actually in most metazoan species) it 

 brings about the formation of a three-layered condition. It is apparent, there- 

 fore, as observed by Pasteels ('37b, p. 464), that "it is impossible to give a 

 general definition of the gastrula stage." It is obvious, also, that one cannot 

 define gastrulation in terms of simple invagination, delamination, or the pro- 

 duction of a two-layered condition. Many processes, involving intricate move- 

 ments of cell groups, occur as outlined in the succeeding pages of this chapter. 

 Relative to the process of gastrulation and later development, emphasis 

 should be placed upon the importance of the blastocoel. The latter takes its 

 origin largely by the movement of groups of cells in relation to one another 

 during cleavage and blastulation. Therefore, we may enumerate the follow- 

 ing events related to the blastocoel during the early phases of embryonic 

 development: 



( 1 ) The blastocoel is associated with those movements in the developing 

 blastula which produce the specific cellular configuration of the ma- 

 ture blastula; 



(2) during gastrulation, it enables the various, presumptive organ-forming 

 areas of the blastula to be rearranged and to migrate into the particular 

 areas which permit their ready organization and axiation into the 

 scheme of the body form of the particular species; and 



(3) in the period of development immediately following gastrulation, it 

 affords the initial space necessary for the tubulation of the major, 

 organ-forming areas. 



The events mentioned in (3) will be described in Chapter 10. 



2. Primitive Vertebrate Body Plan in Relation to the 

 Process of Gastrulation 



In the animal kingdom, each of the major animal groupings has a specific 

 body plan. In the phylum, Chordata, the cephalochordate, Amphioxus, and 

 the vertebrates possess such a plan. It is necessary at this point to review 

 briefly the rudiments of this primitive or basic body plan. 



a. Fundamental Body Plan of the Vertebrate Animal 



The vertebrate body essentially is a cylindrical structure with a head or 

 cephalic end, a middle trunk region, and a tail or caudal end. The dorsum 

 or dorsal region is the uppermost aspect, while the venter or belly lies below. 

 Also, the body as a whole may be slightly compressed laterally. Viewed in 

 transverse section, the body is composed basically of five hollow tubes, par- 

 ticularly in the trunk area. The epidermal tube forms the exterior and within 

 the latter are placed the neural, enteric, and two mesodermal tubes, all oriented 

 around the median skeletal axis or notochord as indicated in figures 188C 

 and 217G and N. 



