PART III 



Tne Development or Primitive 

 EmDryonic Body Form 



The general procedures leading to the development of primitive embryonic body form 

 in the chordate group of animals are: 



(1) Cleavage. Cleavage is the division of the egg into progressively smaller cellular 

 units, the blastomeres (Chap. 6). 



(2) Blastulation. Blastulation results in the formation of the blastula. The blastula 

 is composed of a cellular blastoderm in relation to a fluid-filled cavity, the blaslocoel. 

 The blastoderm of the late blastula is composed of neural, epidermal, notoohordal, 

 mesodermal, and entodermal major presumptive organ-forming areas. In the phylum 

 Chordata, the notochordal area is the central region around which the other areas are 

 oriented (Chap. 7). The major presumptive organ-forming areas of the late blastula 

 exist in various degrees of differentiation (Chap. 8). 



(3) Castrulation. This is the process which effects a reorientation of the presumptive 

 organ-forming areas and brings about their axiation antero-posteriorly in relation to the 

 notochordal axis and the future embryonic body (Chap. 9). During gastrulation the 

 major organ-forming areas are subdivided into minor areas or fields, each field being 

 restricted to the development of a particular organ or part. (Pp. 378, 446, 447. 



(4) Following gastrulation, the next step in the development of embryonic body form 

 is tubulation and extension of the major organ-forming areas (Chap. 10). 



(5) As tubulation and extension of the organ-forming areas is effected, the basic or 

 fundamental conditions of the future organ systems are established, resulting in the 

 development of primitive body form. As the development of various vertebrate embryos 

 is strikingly similar up to this point, the primitive embryonic body forms of all vertebrates 

 resemble each other (Chap. 11). 



In the drawings presented in Part III, the following scheme for designating the major 

 organ-forming areas existing within the three germ layers is adhered to: 



EPIDERMAL ECTODERM 



NEURAL ECTODERM 



MESODERM 



ENTODERM 



NOTOCHORO 



PRE-CHORDAL PLATE 



PERIBLAST 



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