EPITHELIUM 51 



in a direction opposite to that of the effective stroke. The cilia in man 

 produce currents toward the outlets of the body. In the uterine tube 

 the stroke is toward the uterus, presumably favoring the passage of the 

 ova, but the spermatozoa ascend this tube against the current. 



The structure of cilia, because of their small size, is difficult to deter- 

 mine, but in many cases a differentiation between the exoplasm and 

 endoplasm has been observed. The simplest cilia, as shown in the 

 diagram (Fig. 40, a), are essentially permanent pseudopodia, with con- 

 tractile sheaths and fluid contents. They may develop very rapidly 

 in the protozoa. Thus Prowazek has seen processes grow out in eight 

 minutes, which were then vibrating 19 times in 20 seconds. Schafer 



PIG. 40. 



a, b, c, Diagrams to illustrate the structure 

 of cilia. (After Williams.) 



FIG. 41. 



Diagram of a ciliated cell (after Prenant) , 

 showing yibratile cilia; b, cells of the 

 human epididymis (after Fuchs), showing 

 non-motile cilia. 



considers that cilia are primarily pseudopodia, and that their motion 

 is caused by the alternating ingress and egress of fluid to and from the 

 central part, due to variations in the surface tension. 



Many cilia, however, appear to contain more or less solid axial rods, 

 which generally proceed from round basal bodies resembling centrosomes. 

 That these bodies arise from the centrosome has recently been denied. 

 Sometimes the bodies are double, and extensions from them downward 

 into the cytoplasm may occasionally be observed (Fig. 41, a). These 

 roots approach one another beside the nucleus, and it has been discussed 

 whether or not they unite. The roots, and portions of the cytoplasmic 

 reticulum at right angles to the shafts of the cilia, have been thought 

 to act as levers. Others conjecture that the central shaft is a supporting 

 structure, perhaps elastic, which is surrounded by a contractile sheath. 

 The contractile elements may extend the whole length of the cilium or be ' 

 confined to its base, as indicated in the diagram (Fig. 40, b and c). If 



