MUSEUM OF COMPARATIVE ZOOLOGY. 31 



head, shaft, and root. These names may be applied not inappropriately 

 to the three regions of a villus. The peripheral portion or head (cap.), 

 embracing one fourth, or sometimes as much as one third, of the original 

 thickness of the layer, is distinctly prismatic and highly refractive ; its 

 sides are parallel, and it is little affected by the acid, so that, although it 

 increases very slightly in size, it still retains to some extent its angular 

 form. Its free end is always more or less rounded (Plate I. Figs. 4, 9, i). 

 Following this terminal head, and marked off from it by a slight con- 

 striction, comes the stalk (Fig. 9, i, pd.), a long, also prismatic fibre, 

 which is less highly refractive than the head, and is so crowded as to be 

 folded back and forth, thus giving to it the appearance of a spiral spring. 

 In fact, many of the fibres are coiled into a tolerably regular spiral, but 

 the majority are simply folded irregularly, evidently being accommo- 

 dated to the space most available. 



Through the action of acid these stalks begin to swell, and some of 

 them — since they are affected more promptly than others — cause an 

 earlier protrusion of the corresponding prismatic heads (Plate I. Fig. 4). 

 The increase in the thickness of the layer — which soon reaches twice 

 its original dimensions — is due almost entirely to the swelling of these 

 deeper portions of the villi. When isolated, they may in some cases be 

 elongated to ten or twelve times the length of the coils which they at 

 first formed (a, b, Fig. 9, Plate I.). A portion of this elongation is due 

 simply to the unfolding of the compressed stalks ; but ultimately in 

 proportion as it elongates the stalk becomes more attenuated. It often 

 happens in this process that different portions of the stalk are at first 

 unequally affected. Usually it is the deeper portion which is first to 

 uncoil and to become attenuated (n, Fig. 9). When fully extended the 

 stalk is slightly tapering, being narrowest at a little distance from its 

 basal or root end, and although generally quite uniform in calibre, it 

 occasionally exhibits varicosities. In many cases the isolated villi (Fig. 

 9, g, h) appear as though temporarily prevented from straightening out 

 because of delicate longitudinal structures of a band-like appearance. 

 The aspect of the stalk is then remarkably similar to the pouched con- 

 dition of the mammalian colon, to the longitudinal muscles of which 

 these band-like structures correspond. The apparent " bagging " is 

 usually all in one direction, namely, toward the attached end of the 

 villus (h, Fig. 9). The basal end or root (rx.) appears to terminate 

 regularly in a number (3-9) of tapering root-like diverging prolonga- 

 tions (Fig. 9, /, g, i, k), which are often apparently connected with each 

 other by membranous expansions of the basal portion of the villus. 



