AMPLIFICATION OF EPITHELIAL SURFACES 51 



an angle to these folds, which are thereby determined as ridges (Fig. 55). 

 This calls our attention to the fact that the folds in Figure 53 represent 

 either ridges, or grooves, or both, as the case may be determined by other 

 circumstances. 



The placing of these two folds at an angle to each other leaves a 

 series of pockets between them which are called invaginations. As 

 both sets of folds are formed at the same time, we are not accustomed 

 to think of them as such, but to direct our attention to the points which 

 appear to be, and in most cases are, depressions from the surface. If they 

 do originate between the folds, they subsequently are extended by a real 

 inward growth or invagination. 



A good example of such a structure may be studied in the stomach of a 

 common carp. In a fish of moderate or small size the origin of these in- 

 vaginations can be traced to longitudinal corrugations of the surface, 

 as seen lower down in the intestine, and thus we can realize the relations 

 of these surfaces. An examination with a strong lens of these various 

 surfaces on freshly killed material, washed and treated with some hard- 

 ening reagent, should follow or accompany the microscopic work. 



The last specimen to be studied is the duodenum of some small 

 mammal that has been carefully hardened, and bulk-stained, and cut. 

 This should be studied in sections cut in the vertical and horizontal 

 planes as well as one cut in an oblique plane. 



We understand that a section of an organ is only an image of it in 

 one plane. For this reason a perfect vertical section of either a corru- 

 gation, or an evagination, or an invagination, cannot be distinguished 

 from a similar section of the other two. We may also see how a section 

 of any one of these will reveal its true nature by many slight departures 

 from a typical form. Also how it may deceive one by sections taken in 

 the valleys or through the ridges of one or the other of them. 



In considering any amplification of an epithelial surface in this work 

 it will be designated as an evagination or an invagination, according to 

 the relations which it bears to its free or distal surface. When the fold 

 moves proximally, and contains a morphological lumen derived from 

 the free surface, it will be called an invagination. When, on the other 

 hand, it moves upward, inclosing a core of the tissues on which it rested, 

 it will be termed an evagination. Thus the optic cup with its stalk will 

 be considered as an invagination, although many embryologists speak of 

 it as an evagination. 



Technic. No special methods for this part. 



LITERATURE 

 Same as for the last part. 



