INTERNAL ANATOMY OE SQUJLLA. 
411 
entire mass of the gland. If Text- fig. 3 be referred to, it will 
be seen that in the early stage of development figured, the 
septum (i.e. the apposed walls) separating the end-sac from 
the kidney is giving rise to an invagination in the same way 
as the other parts of the gland-wall, and hence the boundary 
between these two parts of the gland becomes very sinuous 
in the fully formed organ (PI. 28, fig. 3), though easily dis- 
tinguishable on account of the characteristic features of the 
end-sac cells. The end-sac as a whole is situated on the 
upper and slightly posterior and external lateral aspect of the 
fnlly formed gland, i.e. it retains the position in which it 
arose. The constituent cells of the end-sac of the fully 
formed gland (Text-fig. 5) are somewhat irregular in outline 
and consist of faintly granular cytoplasm, which is difiicult to 
stain, and large nuclei. In close apposition to the nucleus 
of each cell is a large and very characteristic vacuole (F), 
usually much larger than the nucleus. These cells of the end- 
sac differ markedly from those of the gland proper, and the 
two kinds of cells are quite distinct — there are no cells 
transitional in character. The cells of the fully formed end- 
sac also differ greatly from those of the larval end-sac, as will 
be seen below. The blood-sinus, which extends into the 
septum separating the end-sac from the gland proper, is so 
attenuated in the gland of the adult as to be only recognis- 
able in position by the nuclei of the two opposed layers of 
squamous epithelium composing its walls. 
Histology of the Kidney or Gland Proper. 
As already stated, in tlie region of the kidney, the wide 
lumen of the original sac is reduced to slit-like spaces 
situated between the crowded invaginations of the wall 
already described (Text-fig. 4). Idiese thin spaces represent- 
ing the gland lumen can always be distinguished from the 
equally thin extensions of the external hasmocoele by the 
facts that (a) the excretory matter contained in the lumen is 
largely in the form of spherical bubble-like masses (the blood. 
