30 EPITHELIAL TISSUE. 



of the karyokinetic process. Whether these fibers are formed 

 from the attraction-sphere or from the achromatin of the nucleus 

 is unknown. Each of these centrosomes forms a pole of the 

 spindle. The nuclear membrane now disappears, and there is 

 nothing between the protoplasm of the cell and the nuclear matrix. 

 The protoplasm in contact with the nucleus is clear, while that 

 outside of this clear space is granular. In some cells these gran- 

 ules have the appearance of fine fibers radiating from the centro- 

 somes or poles, and constitute the amphiaster. 



The next stage is characterized by the settling of the chromo- 

 somes to the equator of the spindle, where they form a star or 

 aster, which being single is called monaster : this is known as the 

 equatorial stage. 



The chromosomes now separate so as to form two distinct 

 groups, constituting the stage of metakinesis. One group passes 

 to one end or pole and the other to the other, thus forming a star 

 at each end and giving rise to the term diaster or double star. 

 This passage of the chromosome from the equator to the poles is 

 believed to be accomplished by the contraction of the achromatin 

 fibers of the spindle. Thus from the chromoplasm of the nucleus 

 two new nuclei, or daughter-nuclei, are formed, each aster passing 

 into a resting nucleus by a process the reverse of that by which it 

 was formed, through the dispirem stage. A nuclear membrane 

 forms around each new nucleus, and the protoplasm of the original 

 cell subdivides into two, each half enclosing a new nucleus : at the 

 same time the spindle disappears. 



ELEMENTARY TISSUES* 



EPITHELIAL TISSUE. 



Distributed over the surface of the body, lining its many 

 cavities and canals, and in the ducts of glands, epithelium is 

 found of several varieties and arrangement. The varieties are 

 as follows : Pavement or scaly, cubical, columnar, goblet-cell, 

 spheroidal or glandular, and ciliated. 



Pavement or Scaly Epithelium (Fig. 18). As its name 

 implies, the cells of this variety of epithelium are thin and flat, 

 and are arranged like the stones of a pavement. They are bound 

 together by a small amount of cement-substance. They are found 

 in the lung-alveoli, in the ducts of the mammary glands, and in 

 the kidney in the tubes of Henle, and lining Bowman's capsules. 

 These cells are also found covering serous membranes, as the peri- 

 cardium, and lining blood-vessels and lymphatics, and in that case 

 receive the name of endothelium. 



Cubical Epithelium. This kind of epithelium is of a 



