GENERAL PROCESSES 



393 



(3) The engines are further accelerated and the plane is moved down the 

 runway for the take-off into the airy regions. 



Similarly, during cleavage and blastulation, the embryonic machine develops 

 a readiness, elaborates the major, organ-forming areas in their correct posi- 

 tions in the blastula, and taxies into position with its engines warming up, as 

 it were. Once in the position of the mature blastula, the various, major, pre- 

 sumptive organ-forming areas are turned around and reoriented by the gastrula- 

 tive processes, and thus, each major, organ-forming area of the gastrula is 

 placed in readiness for the final developmental surge which results in primitive 

 body formation. During the latter process the major, presumptive organ- 

 forming areas in the vertebrate group are molded into the form of elongated 

 tubular structures with the exception of the notochordal area which forms an 

 elongated skeletal axis. (The latter phenomena are described in Chapter 10.) 



c. Chart of Blastula, Gastrula, and Primitive Body-form Relationships 

 in the Vertebrate Group 

 (Fig. 188) 

 The major, presumptive organ-forming areas are designated by separate 

 numerals. 



Blastula 



Gastrula 



Primitive Body Form 



1. Epidermal crescent 



2. Neural crescent 



3. Entodermal area 



4. Two mesodermal 

 areas 



5. Notochordal crescent 



1. Part of ectodermal layer 



2. Elongated neural plate a 

 part of ectoderm layer 



3. Primitive archenteron in 

 rounded gastrulae, such as 

 frog; archenteric layer in 

 flattened gastrulae, such as 

 chick 



4. Two mesodermal layers on 

 either side of notochord 



5. Elongated band of cells 

 lying between mesodermal 

 layers 



1. External epidermal tube 



2. Dorsally placed neural tube 



3. Primitive gut tube 



Two primitive mesodermal 

 tubes; one along either side 

 of neural tube, notochord, 

 and gut tube; especially true 

 of trunk region 



Rounded rod of cells lying 

 below neural tube and 

 above entodermal or gut 

 tube; these three structures 

 lie in the meson or median 

 plane of the body 



B. General Processes Involved in Gastrulation 



Gastrulation is a nicely integrated, dynamic process; one which is controlled 

 largely by intrinsic (i.e., autonomous) forces bound up in the specific, physico- 

 chemical conditions of the various, presumptive, organ-forming areas of the 

 late blastula and early gastrula. These internal forces in turn are correlated 



