DIGESTION : SECRETION 125 



seen if the specimen is killed and fixed immediately after it has com- 

 menced to feed, that is, before the gut becomes distended with blood. 

 The first figure on Plate XXVI shows it in a specimen of Tabanus 

 removed before the completion of its first meal after hatching and fixed 

 at once. In the section from which this drawing was made there 

 were many places around the periphery where the layer could not be 

 distinguished. 



The process of secretion of digestive fluid by the cells of the mid-gut in 

 blood-sucking flies is similar to that which has been described for solid 

 feeders. Previous to and during digestion globules 



appear in the protoplasm between the nucleus and the Secretlon and 



. r excretion 



striated border, and gradually increase in size, the 



smaller ones coalescing to form large globules near the free border, till 

 they distend the cell wall, causing it to bulge into the lumen of the gut. 

 The striated border then gives way at a weak point, and a mass of 

 globules is extruded. In many cases the orifice through which the 

 globules pass is a narrow one, so that when the cell is killed in the act of 

 getting rid of its contents, as in some of those in the figure, the mass is 

 flask-shaped, and still remains attached to the cell by a narrow neck which 

 passes through the aperture in the striated border, the remainder of the 

 mass having rounded off as soon as it became free. In other cases 

 the whole of the border seems to give way, and a shower of globules 

 only loosely attached to one another is set free. During this process 

 the cell itself alters in shape, probably on account of the state of tension 

 which exists in its interior. The nucleus moves nearer to the lumen and 

 the whole cell takes on a more conical shape ; it remains attached to the 

 basal membrane by a long and narrow basal process, while the part 

 towards the lumen is correspondingly swollen, and may project into 

 the lumen beyond its fellows. 



In some cases, but not by any means in all, the globules take with 

 them a part of the protoplasm of the cell, including the nucleus. When 

 this occurs the nucleus is always in a condition of degeneration, as is 

 evidenced by the readiness and diffuseness with which it stains, and the 

 large irregular clumps into which its chromatin is collected. Such nuclei 

 in all stages of degeneration can be found free in the lumen in sections 

 of the gut, and can often be seen in the act of leaving the epithelium. 



In the Muscid flies one finds, in most sections, cells at all stages of 

 this process, whether the gut is distended with blood or not ; a row of 

 normal resting cells is seldom seen. The conditions found in the biting 

 flies are approximately the same as in Musca, although their feeding 



