182 
F. M. BALFOUR. 
out it is formed from a solid cord of the cells of the dorsal 
region. The peripheral layer of this cord gives rise to the 
walls of the heart, while the central cells become converted 
into the corpuscles of the blood. 
The rudiment of the heart is in contact with the epiblast 
above, and there is no greater evidence of its being derived 
from the splanchnic than from the somatic mesoblast ; it is, 
in fact, formed before the dorsal mesoblast has become dif- 
ferentiated into two layers. 
In the abdomen three or four transverse septa, derived 
from the splanchnic mesoblast, grow a short way into the 
yolk. They become more conspicuous during the succeed- 
ing stage, and are spoken of in detail in the description of 
that stage. In the anterior part of the thorax a longitu- 
dinal and vertical septum is formed, which grows downwards 
from the median dorsal line, and divides the yolk in this 
region into two parts. In this septum there is formed at a 
later stage a vertical muscle attached to the suctorial part 
of the stomodseum. 
The mesoblastic somites of the earlier stage are but little 
modified ; and there are still prolongations of the body 
cavity into the limbs (PI. XXI, fig. 18). 
The lateral parts of the ventral nerve cords are now at 
their maximum of separation (PI. XXI, fig. 18, v, g). 
Considerable differentiation has already set in in the con- 
stitution of the ganglia themselves, which are composed of 
an outer mass of ganglion cells enclosing a kernel of nerve 
fibres, which lie on the inner side and connect the successive 
ganglia. There are still distinct thoracic and abdominal 
ganglia for each segment, and there is also a pair of separate 
ganglion for the chelicerse, which assists, however, in forming 
the oesophageal commissures. 
The thickenings of the praeoral lobe which form the supra- 
cesophageal ganglia are nearly though not quite separated from 
the epiblast. The semicircular grooves of the earlier stages 
are now deeper than before, and are well shown in sections 
nearly parallel to the outer anterior surface of the ganglion 
(PI. XXI, fig. 19). The supra-oesophageal ganglia are still 
entirely formed of undifferentiated cells, and are without 
commissural tissue like that present in the ventral ganglia. 
The stomodeeum has considerably increased in length, and 
the proctodeeurn has become formed as a short, posteriorly 
directed involution of the epiblast. I have seen traces of 
what I believe to be two outgrowths from it, which form 
the Malpighian bodies. 
The next stage constitutes (PI. XIX, fig. 9) the last which 
i 
