INTRODUCTION 



655 



Fig. 311. (A) Diagram showing basic mesenchymal packing tissue around the various 

 body tubes and notochord. (B) Contribution of embryonic mesenchyme to adult 

 skeletal tissue. 



2. Generalized or Basic Embryonic Skeleton; 

 Its Origin and Significance 



a. Basic Condition of the Skeletal System 



The generalized or basic skeleton of the embryo which has achieved primi- 

 tive body form is composed of the notochord or primitive skeletal axis, 

 together with the mass of mesenchyme which comes to fill the spaces between 

 the epidermal, neural, enteric, mesodermal, and primitive circulatory tubes. 

 Because of the delicate nature of the mesenchymal cells and the coagulable 

 intercellular substance between them, this primitive skeleton sometimes is re- 

 ferred to as the "ghost skeleton" (fig. 311 A). 



b. Origin of the Primitive Ghost Skeleton 



1) Notochord and Subnotochordal Rod. As observed in Chapters 9 and 

 10, the notochord becomes segregated as a distinct entity during gastrulation 

 and embryonic body formation. It soon comes to form a rod-like structure, 

 surrounded by a primitive notochordal membrane. The notochordal axis ex- 

 tends from the pituitary body (hypophysis) and diencephalic region of the 

 brain caudally to the end of the tail (fig. 217). In many of the lower verte- 

 brates, a second rod of cells, the hypochord or subnotochordal rod, evaginates 

 and segregates from the roof of the gut in the trunk region of the embryo 

 during tubulation and early body-form development; it comes to lie immedi- 

 ately below the notochord (fig. 228). The subnotochoral rod soon degenerates. 



