J^OTES ON THE DEVELOPMENT OF THE ARANEINA. 179 
The nuclei of the yolk have increased in number, but the 
yolk, in other respects, retains its earlier characters. 
The next period in the development is that in which the 
body of the embryo gradually acquires the adult form. The 
most important event which takes place during this period 
is the development of the dorsal region of the embryo, which, 
up to its commencement, is practically non-existent. As a 
consequence of the development of the dorsal region, the 
embryo, which has hitherto had what may be called a dorsal 
flexure, gradually unrolls itself, and acquires a ventral 
flexure. This change in the flexure of the embryo is in 
appearance a rather complicated phenomenon, and has been 
somewhat dififerently described by the two naturalists who 
have studied it in recent times. 
For Claparede the prime cause of the change of flexure 
is the translation dorsalwards of the limbs. He compares 
the dorsal region of the embryo to the arc of a circle, the 
two ends of which are united by a cord formed by the line 
of insertion of the limbs. He points out that if you bring 
the middle of the cord, so stretched between the two ends of 
the arc, nearer to the summit of the arc, you necessarily 
cause the two ends of the arch to approach each other, or, in 
other words, if the insertion of the limbs is drawn up dor- 
sally, the head and tail must approach each other 
ventrally. 
Barrois takes quite a different view to that of Claparede, 
which will perhaps be best understood if I quote a transla- 
tion of his own words. He says: ^^At the period of the 
last stage of the embryonic band (the stage represented in 
PI. XX, fig. 7, in the present paper) this latter completely 
encircles the egg, and its posterior extremity nearly ap- 
proaches the cephalic region. Finally, the germinal bands, 
where they unite at the anal lobe (placed above on the 
dorsal surface), form between them a very acute angle. 
During the following stages one observes the anal segment 
separate further and further from the cephalic region, and 
approach nearer and nearer to the ventral region. This dis- 
placement of the anal segment determines, in its turn, a 
modification in the divergence of the anal bands ; the angle 
which they form at their junction tends to become more 
obtuse. The same processes continue regularly till the anal 
segment comes to occupy the opposite extremity to the 
cephalic region, a period at which the two germinal bands 
are placed in the same plane and the two sides of the obtuse 
angle end by meeting in a straight line. If we suppose a 
continuation of the same phenomenon it is clear that the 
