AMPLIFICATION OF EPITHELIAL SURFACES 49 



surface. But it is an histological process that does not increase the 

 morphological or primary surface, and all that it needs is a little greater 

 thickness of this primary surface. 



There are three stages of this process, resulting in three conditions. 

 They are corrugation, evagination, and invagination, using these terms 

 in a special and histological sense. 



The first of these, corrugation, is the simplest, and consists of the throw- 

 ing of an epithelial surface into a series of parallel folds on the connect- 

 ive tissue base (Fig. 53). This 

 folding is not necessarily vis- 

 ible to the naked eye, and it 

 may or may not yield and be- 

 come flat when the surface is 

 stretched. 



Good examples of a real 

 corrugation are somewhat rare 

 in the adult organism owing 

 to the fact that this process 

 is usually but the beginning 



Of the Other tWO. FIG. 53. Diagram of a corrugated epithelial surface. 



The corrugated side of the foot in some lamellibranch mollusks shows 

 a splendid example of corrugation, with the limitation that it is some- 

 times partly obliterated by stretching, when the foot is extended. Sec- 

 tions at right angles to these corrugations are diagrammatically shown in 

 Figure 53. It will be noticed here that one cannot say whether folds 

 have been thrown up from, or depressions have been made in, the sur- 

 face. A second form of corrugation can be seen in the embryonic stages 

 of the small intestine of vertebrates. This stage is transitory, however, 

 and soon passes into the next form of amplification. 



This next form of amplification will be either that of evagination or 

 invagination, and is determined by the way in which a cross folding is 

 brought about. Figure 53 shows a simple corrugation, and we must now 

 imagine that this folding has not sufficiently increased the surface and 

 that a second corrugation is to be superimposed upon the first to increase 

 its surface, not directly, for it can be seen in the figures that the surface 

 is not mathematically increased, but by putting it in a form in which the 

 specialization can be carried much farther than a plain corrugation could 

 be. A cross corrugated surface would not be torn so easily, and the 

 lumen would allow fluids to pass more easily through it than if it were 

 lined with deep folds. 



This second form of amplification is produced by making a series of 

 depressions at right angles to the original folds (Fig. 54). These may 

 be said to determine the original folds as depressions rather than ridges, 



