460 



DEVELOPMENT OF PRIMITIVE BODY FORM 



Regardless of differences, however, all vertebrate gastrulae, rounded and 

 flattened, possess three fundamental or basic regions, to wit, ( 1 ) a cephalic 

 or head region, containing the rudiments of the future head and pharyngeal 

 structures, (2) a trunk region, wherein lie the undeveloped fundaments of 

 the trunk, and (3) an end-bud or tail rudiment, containing the possibilities 

 of the future tail. 



4. Developmental Processes Which Accomplish Tubulation 



a. Immediate Processes 



The term, immediate processes, signifies the events which actually produce 

 the hollow tubular condition. In the case of the epidermal, enteric, and neural 

 tubulations, the immediate process is mainly one of folding the particular, 



BRAIN AREA ^^7^^r^&?^^^ 



HEAD REGION J^^ 7^ * 



SENSORY PLATE 



GILL - PLATE ■■ 

 AREA 



TRUNK REGION ■ 



NEURAL FO 



TAIL REGION 

 PRE-CHORDAL PLATE 



NEURAL ECTODERM 

 EPIDERMAL 



ME30DE RM 



ECTODERM 



COELOMIC SPACE 



ENTODERM 



NOTO CHORD 

 NEURAL 

 E C T D E R M 



VENTRAL 

 MESODERM 



Fig. 219. Relationships of the major presumptive organ-forming areas at the end of 

 gastrulation in the anuran amphibia. (A) External view of gastrula, showing the ecto- 

 dermal layer composed of presumptive epidermis (white) and presumptive neural plate 

 (black), as viewed from the dorsal aspect. (B) Diagrammatic median sagittal section 

 of condition shown in (A). (C) Same as (B), showing major organ-forming areas. 

 (D) Section through middorsal area of conditions (B) and (C), a short distance caudal 

 to foregut and pre-chordal plate region. Observe that the notochord occupies the mid- 

 dorsal area of the gut roof. 



