522 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS 



specific developmental procedures which change the generalized conditions 

 into specific conditions. 



The laws of von Baer ( 1828-1837, Part I, p. 224) may be stated as follows: 



(a) The general features of a large group of animals appear earlier in de- 

 velopment than do the special features; 



(b) after the more general structures are established, less general structures 

 arise, and so on until the most special feature appears; 



(c) each embryo of a given adult form of animal, instead of passing 

 through or resembling the adult forms of lower members of the group, 

 diverges from the adult forms, because 



(d) the embryo of a higher animal species resembles only the embryo of 

 the lower animal species, not the adult form of the lower species. 



D. Contributions of the Mesoderm to Primitive Body Formation and 



Later Development 



The mesoderm is most important to the developing architecture of the 

 body. Because the mesoderm enters so extensively into the structure of the 

 many organs of the developing embryo, it is well to point out further the 

 sources of mesoderm and to delineate the structures and parts arising from 

 this tissue. 



1. Types of Mesodermal Cells 



Most of the mesoderm of the early embryo exists in the form of epithelium 

 (see p. 519). As development proceeds, much of the mesoderm loses the close 

 arrangement characteristic of epithelium. In doing so, the cells separate and 

 assume a loose connection. They also may change their shapes, appearing 

 stellate, oval, or irregular, and may wander to distant parts of the body. This 

 loosely aggregated condition of mesoderm forms the primitive mesenchyme. 

 Though most of the mesoderm becomes transformed into mesenchyme, the 

 inner layer of cells of the original hypomeric portion of the mesodermal tubes 

 retains a flattened, cohesive pattern, described as mesothelium. Mesothelium 

 comes to line the various body cavities, for these cavities are derived directly 

 from the hypomeric areas of the mesodermal tubes (Chap. 20). 



2. Origin of the Mesoderm of the Head Region 



The primary cephalic outgrowth (Chap. 10), which later forms the head 

 structures, contains two basic regions, namely, the head proper and the 

 pharyngeal or branchial region. During its early development, the heart lies 

 at the ventro-caudal extremity of the general head region; it recedes gradually 

 backward as the head and branchial structures develop. The exact origin of 

 the mesoderm which comes to occupy the head proper and pharyngeal areas 

 varies in different gnathostomous vertebrates. The general sources of the head 

 mesoderm may be described in the following manner. 



