694 Laura Florence 



and in accordance with the mode of Hfe of the insect it appears to be 

 always in a state of activity. In the study of the epitheHum many series 

 of sections of the ahmentary canal have been made, at intervals of from 

 half an hour and one hour after the time of feeding up to twelve hours, 

 by which time the stomach in captive specimens appeared to be empty of 

 blood. Sections have also been made of lice starved to the point of death. 

 The epithelial cells vary in outline according to their state of activity. In 

 the resting stage (Plate LXIII, 1) they are flattened and extend farthest 

 into the lumen in the region of their nuclei. During absorption the 

 individual cells expand until they appear cuboidal, and during secretion 

 the free ends of the cells, where the ball of secretion accmnulates, become 

 subcircular. These secreting cells show great variation in the degree 

 to which they extend into the Imnen. They may remain attached to 

 the basement membrane by a broad base, or they may be greatty atten- 

 uated and apparently attached to the membrane by a very narrow base, 

 and in sections blood is seen extending between the individual cells 

 (Plate LXIII, 1). In no case has a definite cell wall been found between 

 any two cells, and the whole appearance suggests a syncytium ; but further 

 proof would be necessary before the acceptance of this view. Each 

 cell has a large oval nucleus with a subcentral nucleolus surrounded by 

 irregularly scattered chromatin granules. There is considerable variation 

 in the position of the nucleus in the cell, and this, in addition to the 

 irregularity of the cells, gives the effect of a several-layered epithehum 

 (Plate XLIII, 1 and 2). In most cases the nucleus is seen lying in the 

 cytoplasm immediately behind or to one side of the secretion products, and 

 on their excretion remains intact, but in a few cases the nucleus has been 

 seen to be carried along with the secretion (Plate LXIII, 2). In the latter 

 case the death of the cell must follow, and the question of its replacement 

 arises. In many insects a regular destruction of the epithehmn takes 

 place and new cells are formed from regenerative centers, or nidi; but no 

 such structures are present in the hog louse, nor has Silcora (1916:65) 

 found them in Pediculus vedimenti. Nuclear division has not been seen 

 taking place in the epithelimn, but just within the basement membrane 

 at the base of and between some of the epithelial cells he single, very small 

 nuclei, each hardly more than a nucleolus, definite^ surrounded by a 

 small amount of protoplasm; and these may be the source of the new 

 epithelial cells. A similar condition was described by Van Gehuchten 



