1918] Alimentary Canal of Altica Larva ' 293 



The Mid-Intestine, 

 the first division of the ventriculus. 



Striated border. The epithelium of the mid-intestine never 

 supports an intima in insects, but instead is clothed with a 

 striated border, which is very characteristic of this region. 

 This striated border is usually considered as a vestige of a 

 formerly ciliated condition. In this species, the striated border 

 is always conspicuous and well developed. The striae are close 

 together, and are not always sharply separated from the 

 cytoplasm interiorly. 



Epithelium. The epithelium is of the columnar type, and 

 the cells are very clearly defined. They vary greatly in size 

 and shape; they may be almost cuboidal, or more than twice 

 as high as wide, probably depending upon their physiological 

 condition. In general, the more anterior cells are the higher. 

 The nuclei are large, median, and almost round; they are quite 

 densely filled with coarse chromatin granules, and there are 

 from one to three larger granules, probably nucleoli. The 

 cytoplasm is non-granular, but is usually more or less vacuolar. 



The secretion is merocrine. In the resting state the cyto- 

 plasm becomes charged with vacuoles, which pass to the outer 

 margin of the cell, beneath the striated border, where they 

 merge into larger drops. At the time of the secretion these 

 cytoplasmic balls pass out of the cell through the striated 

 border, apparently pushing away the striae on either side. In 

 the process of being discharged, they remain more or less 

 attenuated on the inner end. but they become spherical as soon 

 as they are free in the lumen of the canal. They stain with 

 eosin, and appear homogeneous. 



The first division of the mid-intestine is shown in figure 11. 



Replacement cells. During the pupal and prepupal period 

 of inost, if not all, insects, the epithelium of the mid-intestine 

 breaks down completely, and a new epithelium is built up from 

 little cells or groups of cells, which lie at the base of the ordinar\^ 

 epithelial cells. In Altica bimarginata at least, they seem to 

 remain quiescent during larval life, but begin to proliferate 

 actively during the prepupal period, in which the old larval 

 epithelium breaks down, and a new epithelium is developed, 

 apparently out of these replacement cells. They are apparent 

 in sections through larvcC which have just hatched from the egg. 



