520 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS 



veloping head and tail regions, however, mesoderm is present in the form of 

 loosely aggregated cells, known as mesenchyme. While the cells of the epi- 

 thelial variety are rounded or cuboidal in shape with little intercellular sub- 

 stance or space between the cells, mesenchymal cells tend to assume stellate 

 forms and to have a large amount of intercellular substance between them. 

 The primitive vascular or blood tubes are composed of epithelium in the sense 

 that the cells are closely arranged. However, as these cells are flattened and 

 show specific peculiarities of structure, this tissue is referred to as endothelium. 

 Also, while the cells of the early neural tube show the typical epithelial fea- 

 tures, they soon undergo marked changes characteristic of developing neural 

 tissue. The primitive or generalized, embryonic body thus is composed of 

 four fundamental tissues, viz., epithelial, mesenchymal, endothelial, and neural 

 tissues. 



B. Transformation of the Primitive Body Tubes into the Fundamental 



or Basic Condition of the Various Organ Systems 



Present in the Primitive Embryonic Body 



1. Processes Involved in Basic System Formation 



As the primitive body tubes (epidermal, neural, enteric, and mesodermal) 

 are established, they are modified gradually to form the basis for the various 

 organ systems. While the notochordal axis is not in the form of a tube, it also 

 undergoes changes during this period. The morphological alterations, which 

 transform the primitive body tubes into the basic or fundamental structural 

 conditions of the systems, consist of the following: 



(a) extension and growth of the body tubes, 



(b) saccular outgrowths (evaginations) and ingrowths (invaginations) 

 from restricted areas of the tubes, 



(c) cellular migrations away from the primitive tubes to other tubes and 

 to the spaces between the tubes, and 



(d) unequal growth of different areas along the tubes. 



As a result of these changes, the primitive neural, epidermal, enteric, and 

 mesodermal tubes, together with the capillaries or blood tubes and the noto- 

 chord, experience a state of gradual differentiation which is directed toward 

 the production of the particular adult system to be derived from these re- 

 spective basic structures. The primitive body tubes, the primitive blood capil- 

 laries, and the notochord thus come to form the basic morphological condi- 

 tions of the future organ systems. The basic structural conditions of the various 

 systems are described in Chapters 12 to 21. 



2. Fundamental Similarity of Early Organ Systems 



The general form and structure of each primitive embryonic system, as it 

 begins to develop in one vertebrate species, exhibits a striking resemblance 



