298 



HISTOLOGY 



this region they have 

 greatly increased in diam- 

 eter. The secretion par- 

 ticles of these cells are 

 large and oval (Fig. 265, B). 

 The epithelium of the 

 intestine of the hornet 

 may be taken as an ex- 

 ample of digestive tissue 

 among the insects. The 

 epithelium is thrown into 

 wavelike folds or corruga- 

 tions. The columnar cells 

 vary in height. They are 

 tallest on the ridges and 

 smallest in the grooves. 

 The fundus of each groove 

 forms a center from which 

 new cells appear to be 

 proliferated. The cyto- 

 plasm is reticular to alve- 

 olar. The nuclei lie near 

 the bases of the cells (Fig. 

 266). 



We shall take the gastric gland of the crayfish as an example of 

 the more highly specialized digestive tissues of the Arthropoda. This 



FIG. 266. Lower part of one of the fold-glands in the 

 intestine (ventriculus) of a hornet, "Scolia dubia. ge.c., 

 germinal cells; mus. /., muscle fibers in longitudinal 

 and transverse section. Upper cells are absorptive. X 

 75- 



FlG. 267. A, transverse section of the middle part of a tubule of the crayfish's digestive gland; 

 B, longitudinal section of the tip of same showing the growth point. Many vacuoles shown 

 with or without contained secretion products. X 1000. 



gland is composed of a very extensive system of tubules. Each tubule is 

 lined with a columnar epithelium which is furnished with a membrana 



