HISTOLOGY 



The result, then, is a surface marked into a series of elevations by two 

 series of grooves at right angles to each other, and with the areas thus 



marked off rising into a 

 set of elevations. These 

 elevations are known as 

 ei 'aginations, and they may 

 be very long and close set 

 or fewer in number. They 

 are not always formed by 

 two successive groovings 

 of the surface, but may 

 arise from it simply as 

 outgrowths. 



In the case of the ex- 



FIG. S4 .-Diagram of an evaginated epithelial surface. am P le that W6 sha11 Stud y> 



the villi of the small intes- 

 tine in some vertebrates, the process of formation of the evaginations 

 is approximally the same as the purely theoretical discussion outlined 

 above has indicated, the first amplification of the surface of the lower 

 small intestine in man being in the shape of grooves that are roughly 

 parallel and not straight, but arranged in a sinous course. The division 

 of the ridges lying between these grooves into papillae by a second set of 

 grooves at an angle to the first begins at an early date, in fact, before the 

 first grooves are fully formed. In the upper large intestine of man this 

 process can be best studied owing to the fewness of the evaginations, but 

 after the villi are formed, they pass away and are not seen in the adult 

 organ. 



This furnishes an ontogenetic case of a process that may also be 

 seen in a taxonomic series. 

 The early taxonomic example 

 is the adult intestine in many 

 fishes, in which the amplifica- 

 tion has only proceeded as far 

 as a grooving. 



On the whole this process 

 of amplification by evagination 

 is a rare one. The mechanical 

 and physiological advantage 

 seems to be all in favor of the 

 third, which is that of imagi- 

 nation. 



This process consists (theoretically and many times actually) of the 

 raising of a series of ridges across the surface of the original folds, at 



FIG. 55. Diagram of an invaginated epithelial sur- 

 face. 



