.168 
F. M. BALFOUR. 
minal layers and organs accord in most points with my 
own. 
The development of the Araneina may be divided into 
four periods : (1) the segmentation ; (2) the period from the 
close of the segmentation up to the period when the segments 
commence to be formed ; (3) the period from the com- 
mencing formation of the segments to the development of the 
full number of limbs ; (4) the subsequent stages up to the 
attainment of the adult form. 
In my earliest stage the segmentation was already com- 
pleted, and the embryo was formed of a single layer of large 
flattened cells enveloping a central mass of polygonal yolk- 
segments. 
Each yolk-segment is formed of a number of large clear 
somewhat oval yolk-spherules. In hardened specimens the 
yolk-spherules become polygonal, and in ova treated with 
hot water prior to preservation are not unfrequently broken 
up. Amongst the yolk-segments are placed a fair number, 
of nucleated bodies of a very characteristic appearance. 
Each of them is formed of (1) a large, often angular, nucleus, 
filled with deeply staining bodies (nucleoli ?) . (2) Of a layer 
of protoplasm surrounding the nucleus, prolonged into a 
protoplasmic reticulum. The exact relation of these 
nucleated bodies to the yolk-segments is not very easy to 
make out, but the general tendency of my observations is to 
show (1) that each nucleated body Delongs to a yolk-sphere, 
and (2) that it is generally placed not at the centre, but to 
one side of a yolk-sphere. If the above conclusions are 
correct each complete yolk-segment is a cell, and each such 
cell consists of a normal nucleus, protoplas-m, and yolk- 
spherules. There is a special layer of protoplasm surrounding 
the nucleus, while the remainder of the protoplasm consists 
of a reticulum holding together the yolk -spherules. Yolk- 
cells of this character are seen in PI. XX and XXI, fiers. 
10 — 21 . 
The nuclei of the yolk-cells are probably derived by divi- 
sion from the nuclei of the segmentation rosettes {vide Lud- 
wig, No. 5), and it is probable that they take their origin at 
the time when the superficial layer of protoplasm separates 
from the yolk-columns below to form the blastoderm. 
The protoplasm of the yolk-cells undergoes rapid division, 
as is shewn by the fact that there are often two nucleated 
bodies close together, and sometimes two nuclei in a sino^le 
mass of protoplasm (fig. 10). It is probable that in some 
cases the yolk-spheres divide at the same time as the proto- 
plasm belonging to them; the division of the nucleated 
