TISSUES 



31 



also connective tissue elements for unification and support; also vascular 

 and nervous constituents. Tissues in which the cell boundaries are ab- 

 sent are known as syncytia (Fig. 34). A syncytium may obviously arise 



through nuclear proliferation in the ab- 

 sence of cytoplasmic division,, or as the 

 result of the disappearance of original 

 cell boundaries. We may distinguish 



FIG. 34. A VILLUS OF THE HUMAN 

 PLACENTA, SHOWING A PERIPH- 

 ERAL SYNCYTIUM OF IRREGULAR 

 THICKNESS. 

 The connective tissue inclosed 



by the syncytium contains three 



capillary vessels. Hematein and 



eosin. X 500, 



FIG. 35.- CELLS FROM THE PANCREAS OF 

 NECTURUS, CONTAINING SECRETORY GRAN- 

 ULES AND BASAL ERGASTOPLASMIC FILA- 

 MENTS. (After Matthews.) 



the following fundamental tissues: (a) epithelial; (b) connective; (c) 

 muscular; (d) nervous; and (e) vascular. 



Lymphoid tissue may be regarded as still another fundamental tissue; 



FIG. 36. VARIOUS FORMS OF CELLS. 



a, squamous epithelium from the tongue; b, a columnar cell from the small intestine; 

 c, a polyhedral or spheroidal cell from the liver; d, a smooth muscle cell from the mus- 

 cular coat of the stomach. X 550. 



or it may be included under vascular tissue. In fact from the genetic 

 viewpoint, vascular may be included under connective tissue, since both 

 arise from the mesenchyma. 



