A STUDY OF THE BLOOD OF CPJRTAIN COLEOPTEEA. 159 



on edge, and round, with two polar ])rolong'ations, when viewed 

 from above. They measure in both Dytiscus and Hydro- 

 philns from 17 ju by 19 ju to lb /u by 30 jli. In Dytiscus the 

 cytoplasm of these cells is coarser and more largely vacuolated 

 than in Hydrophilus. The nucleus in Dytiscus has 

 a definite membrane and the chromatin is diffusely and 

 irregularly distributed. Faint strands of linin connect 

 together the chromatin masses. Generally only one nucleolus 

 is present (see figs. 1-4). In Hydrophilus a well-defined 

 nuclear membrane is also present, but otherwise the nucleus 

 is strikingly different in appearance to that of Dytiscus, for 

 instead of being distributed in unequal nmsses, as in the latter 

 insect, the chromatin occurs in the form of about twenty-five 

 to thirty nearly equal-sized aggregations, and these generally 

 appear to be split in one direction, giving the appearance of 

 twin masses of chromatin.^ The linin is inconspicuous (see 

 figs. 11-13). When these cells have ingested foreign particles 

 from the plasma they change their shape, gradually drawing 

 in their polar extensions and becoming more or less round 

 (see figs. 6-9 and. 12-14). Both in the fresh and well-fixed 

 blood of Dytiscus nnd Hydrophilus it can be seen thaD 

 the majority of the phagocytes which contain no food-particles 

 or recent food-vacuoles in their cytoplasm possess the polar 

 prolongations. At all times the phagocytes may exhibit 

 short and thin pseudopodia extruded from various parts of 

 the cytoplasm, but the polar extensions, although of a more 

 permanent nature, are themselves only pseudopodia, and are 

 distinctive ot that phase in the life of the cell in which no 

 ingestion and digestion occur. 



The other kind of cell found in the blood is a small cell, 

 with large nucleus and very little cytoplasm (see figs. 10 and 

 18). These cells, for want of a more convenient term, we 

 designate as small round-cells. As in the case of the phagocytes, 



1 This arrangement of tlie chromatin in twin groups is apjDarently 

 cliaracteristic of the somatic cells of Hydrophilus. It can be seen, 

 for instance, in the Malpigliian tube cells, in the cells of the glands of 

 the mid-gut, and in the spermatogonia. 



