180 
F. M, BALFOUR. 
anal segment will come to occupy a position on the ventral 
surface, and the germinal bands to approach, but in the 
inverse way, so as to form an angle opposite to that which 
they formed at first. This condition ends the process by 
which the posterior extremity of the embryonic band, at first 
directed towards the dorsal side, comes to bend in towards 
the ventral region.” 
Neither of the above explanations is to my mind perfectly 
satisfactory. The whole phenomenon appears to me to be 
very simple, and to be caused by the elongation of the 
dorsal region, ^. e, the region on the dorsal surface between 
the anal and procephalic lobes. Such an elongation neces- 
sarily separates the anal and procephalic lobes ; but, since 
the ventral plate does not become shortened in the process, 
and the embryo cannot straighten itself on account of the 
egg-shell, it necessarily becomes flexed, and such flexure can 
only be what I have already called a ventral flexure. If there 
were but little food yolk this flexure would cause the whole 
embryo to be bent in, so as to have the ventral surface con- 
cave, but instead of this the flexure is confined at first to the 
two bands which form the ventral plate. These bands are 
bent in the natural way (PL XIX, fig. 8, b), but the yolk 
forms a projection, a kind of yolk sack as Barrois calls it, dis- 
tending the thin integument between the two ventral bands. 
This yolk sack is shown in surface view in P1. XIX, fig. 8, 
and in section in PI. XXI, fig. 18. At a later period, when 
the yolk has become largely absorbed in the formation of 
various organs, the true nature of the ventral flexure 
becomes apparent, and the abdomen of the young Spider is 
found to be bent over so as to press against the ventral 
surface of the thorax (PI. XIX, fig. 9). This flexure is 
shown in section in PI. XXI, fig. 21. 
At the earliest stage of this period of which I have ex- 
amples, the dorsal region has somewhat increased, though 
not very much. The limbs have grown very considerably and 
now cross in the middle line. 
The ventral ganglia, though not the supra-oesophageal, 
have become separated from the epiblast. 
The yolk nuclei, each surrounded by protoplasm as before, 
are much more numerous. 
In other respects there are no great changes in the internal 
features. 
In my next stage, represented in PI. XIX, figs. 8 a, and 
8 a very considerable advance has become effected. In 
the first place the dorsal surface'has increased in length to 
rather more than one half the circumference of the ovum. 
