330 FRANCIS O. HOLMES. 



plant and insect flagellates by histological studies have been 

 made. The results have been negative so far as obtaining definite 

 proof of the identity of the two forms is concerned, but such 

 interesting observations have been made during the study that 

 they will be described here. 



The first insects to be sectioned and stained were collected 

 during 1923. At first the flagellates were overlooked, and that 

 for two very good reasons. The principal search was made for 

 them in the intestinal tract, where they do not occur in my 

 material, if indeed they ever occur there. And the salivary gland 

 forms, stained as carefully as they may be, never stand out with 

 the clearness by which those in latex smears are characterized. 

 For the process of drying and staining with Wright's stain, 

 though open to the objection that the nuclear detail is lost, gives 

 bright sharp pictures of the organisms, usually making them 

 much darker than the background. This is far from the case 

 with wet-fixed material, sectioned, and stained even with so good 

 a stain as iron hsematoxylin. By this process the nucleus may 

 be stained with all the desired sharpness, but the background of 

 salivary secretion retains the stain far more than does the 

 flagellate's cytoplasm. So that usually one sees the body of the 

 flagellate only as an unstained area surrounding the nucleus. 

 The wall of the salivary gland also retains the stain so tenaciously 

 that it must usually be left very black that the internal structure 

 of the flagellate may be seen at its best. This makes the whole 

 field very dark, with the object to be examined exceedingly 

 delicate and lightly stained. When these first specimens of 

 Oncopeltus were examined a second time for a different purpose, 

 clusters of the flagellates were noted by chance on the wall of 

 the gland. The slide under observation at the time was stained 

 with a mixture of aniline dyes, but the finding was at once 

 confirmed by iron haematoxylin slides of the same material. 



During 1924 a quantity of material was obtained for sectioning 

 to discover the extent of the infection among the insects during 

 the late season when the plants were becoming more and more 

 widely infected, to determine with certainty that the organ in 

 question was really the salivary gland of the insect and not a 

 salivary receptacle or some other organ, to see whether the 



