36 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
27,35; Plate 6, Fig. 56). ‘They are in the closest apposition with the 
great nerves (n. at.) which supply the first antennae, and they lie on 
the median side of these nerves, but have no connection with them. 
b. Composition.— In entire preparations and on cursory examina- 
tion of sections, the structures may really appear to be vesicles, as 
Claus said; but closer inspection shows that each organ is composed of 
two large cells of about equal size, which are so much flattened against 
each other that they together form an approximately spherical body. 
Each cell contains a fairly large nucleus (Plate 2, Fig. 20; Plate 3, 
Figs. 31, 33) with chromatin in the form of a network. A comparison 
of the dimensions of nuclei and cells in the brain proper with those 
of the organs of Claus, shows that, while the nucleus of a brain cell, 
on the average, is a little larger than a nucleus of one of the cells in an 
organ of Claus, the cells of the latter are several times as large as the 
cells of the brain. ‘The average diameter of nuclei in the cells of the 
brain 1s approximately 9.1 4, while the nuclei of Claus’s organ are on 
the average not over 8.5 4 in diameter. The average diameter of the 
cells in the brain is 11.7 4, while the cells in the organs of Claus average 
30 win diameter. ‘These values have been obtained by careful meas- 
urement of cells and nuclei both in whole preparations and in sections 
cut in three planes. 
ce. Sumilarity in Structure to the Cells of the Median Eye.— The 
most striking thing about the cells of the organs of Claus is their 
resemblance to the cells of the median eye. It may be said that, 
within certain limits, to be later defined, a cell of an organ of Claus 
corresponds in every way with a retinal cell of the median eye. Each 
of the two cells in Claus’s organ is provided with a structure which is 
an exact counterpart of the basal plate in the median eye. It becomes 
red in Mallory’s stain (Plate 5, Fig. 55, la. ba.) and brown or black in 
vom Rath’s mixture (Plate 2, Figs. 11, 12, la. ba.). But in the median 
eye, a basal plate is shared by several cells, whereas in the organs of 
Claus each cell has formed its own basal plate, which covers a portion 
of the periphery of the cell. Sections cut in the three principal planes, 
and whole preparations, show that the basal plate measured in any 
direction occupies about one-third of the entire periphery of its cell 
(cf. Plate 5, Fig. 55, Plate 2, Figs. 18 and 20). As in the case of the 
median eye, the basal plate of the organ of Claus possesses no intrinsic 
structure. The region of the cells occupied by the basal plates is 
also, in part, the region in which the cells are in contact. ‘This gives 
rise to an appearance in darkly stained vom Rath material (shown, 
for example, in Plate 2, Fig. 12 and Plate 3, Fig. 36) which suggests 
