24 NORMAL HISTOLOGY. 



yielding, while with adolescence they may become tough and re- 

 sistant. Accompanying the growth of the tissue, an increase of the 

 intercellular substance usually takes place through the direct or 

 indirect participation of the cells, these latter, in consequence, suf- 

 fering marked reduction in number and size. The younger the 

 mesodermic tissue, the richer is it in cells and the poorer in intercellu- 

 lar substances; conversely, the older the tissue, the more prominent 

 the intercellular substance and less conspicuous the cellular elements. 

 A marked example of this law is presented by tendon, where, in the 

 embryonic condition, the cells constitute the greater bulk of the 

 tissue, while in the adult the intercellular fibrous tissue so overwhelms 

 the cellular elements that reagents are frequently necessary to satis- 

 factorily demonstrate their existence. 



While increase of the intercellular substance usually accompanies 

 the growth of the mesodermic tissues, those derived from the 

 ecto- and entoderm present a marked contrast. In these latter 

 tissues the intercellular constituent is represented by the very scanty 

 cement substance, increase in which occurs only as necessitated by 

 the growth of the surrounding cells, the proportion between the two 

 elements being practically constant throughout life. Instances of 

 this constant relation are seen in the varieties and modifications of 

 the epithelial tissues. 



The primary blastodermic layers ectoderm, mesoderm, and 

 entoderm early exhibit histological differences which suffice to 

 distinguish the one from the other, and especially to indicate, at least 

 in a general manner, the tendency of the outer and inner layers to 



FIG. 15. 



Blnstodermic layers of rabbit embryo : a, ectoderm ; b, entoderm ; c, entodermal cells destined to 

 form notochord ; nt, mesoderm. 



form epithelial structures in contrast to the less compact and more 

 reticular formations of the mesoderm. The epithelia of the genito- 

 urinary tract, however, are marked exceptions in their origin, being 

 derived, as well as the connective and muscular tissues, from the 

 mesoderm, in this respect constituting conspicuous specializations. 



