24 



PSYCHOBIOLOGY 



ing cells; mesenchymal cells, whose branches anastomose, that is, be- 

 come joined together with protoplasmic continuity from cell to cell, form- 

 ing what is called a syncytium. 



From these three layers of the blastoderm develop the following tissues, 

 comprising the body of the adult individual. 



1. Epithelium. This is the tissue that covers surfaces, internal and ex- 

 ternal. The cells are closely packed, and cemented together by substances 

 secreted by themselves. An epithelium may be simple, composed of one 



Fig. 12. Areolar connective tissue ; sub-cutaneous, from rabbit. Highly magnified. 

 (Schafer, Microscopic Anatomy.) The wavy bundles are white fibers; the straight 

 black lines forming an open net-work are elastic fibers. Several types of connective- 

 tissue cells are shown at e, f, g, and v. 



layer of cells; or stratified, composed of several layers [Fig. 10]. The 

 cells from surface view are usually polygonal, and often six-sided. If the 

 depth of the cells is approximately equal to their width, the epithelium 

 and its component cells are both described as cuboidal; if the depth is 

 greater than the width, they are called columnar [Fig. 11] ; if the depth 

 is less than the width, giving the cells a flattened or scale-like form, the 

 term squamous is applied. 



Epithelial cells may become hardened ( ' cornified ' ) , as on the surface 

 of the epidermis, on the nails, and in hair. Other epithelial cells may 



