30 
W. F. PURCELri, 
book, tlins forming a passage for the blood and blood- 
corpuscles {hd.e.) from the one side to the other. All 
mitoses definitely cease in such saccules, although they are 
common enough in the previous stages, as well as in younger 
not yet chitinised saccules of all subsequent stages. The two 
adjacent walls do not, however, lose contact with one another, 
for each cell of a dorsal wall of a saccule (with a few excep- 
tions) remains united with one or two cells of the ventral wall 
of the adjacent saccule by means of a column of protoplasm, 
in the formation of which both or all three cells (ir., fig. 18) 
take part. Owing to the excess of nuclei in the ventral wall 
of the saccnle we often find a column provided with two nuclei 
at its dorsal and one at its ventral end (//., fig. 18), while 
some of the cells of the ventral wall become simple plaster- 
cells unattached to a column {z., fig. 18). Similar double 
nuclei and plaster-cells ai*e rarely found in the dorsal wall of 
a saccnle. This arrangement of the nuclei is retained through 
all subsequent stages up to the adult form, and was found in 
the adults of all other spiders examined.' I also found it in 
embryos of Agelena labyrinthica, and it is evidently 
general amongst Dipnenmonous spiders. 
The nuclei vary greatly in shape. IMany are more or less 
depressed in the plane of the septa, becoming plano-convex 
oi- conical, the plane side facing the chitinous cuticula. 
'Phe cells of the ventral wall of the oldest saccnle (.v. 1) 
require special mention. These also form columns, which 
attach themselves to the body hypodermis, but the cells of 
the latter do not contribute to these structures. The nuclei 
of this saccule are often drawn out in a peculiar way into the 
thinnest part of the ventral columns (fig. 17). Locy, who 
describes these columns, considers them to be probably of a 
muscular nature, but there does not .seem to me to be any 
reason for thinking that they are any more muscular than 
the colmnns of the septa. Their greater length is simply 
explained by the fact that each cell has to form a column, at 
' The plaster-cells were first noticed by Berteanx ('89) in fully 
developed spider’s lun"s. 
