No. 2.] BLATTA AND DORYPHORA. 367 
rapidly by karyokinesis, and might be called ganglioblasts, as the 
products of their divisions reinforce the mass of ganglion cells. 
In a series of sections through the mesothoracic pair of ganglia 
(Figs. 94 to 104) the Mittlestrang may be readily traced. The 
shape of its cross-section varies with the plane of section 
through the ganglion and its cross or longitudinal commis- 
sures. At the two points in the ganglion where the two pairs 
of Punktsubstanz masses fuse to form the commissures c cm, the 
Mittlestrang is in great part obliterated. The median strand is 
largest where the two longitudinal commissures are passing into 
the anterior ends of the three thoracic ganglia. Here they 
persist in the larva, while completely disappearing elsewhere, 
and become converted into the three chitinous furce, cach one of 
which ws gust in front of a thoracic ganglion. 
Figure 105, from a section through the fore end of the meta- 
thoracic ganglion, shows the Mittlestrang portion (#s¢) contin- 
uous with the hypodermis (ect) and broadening out into the 
furca (f) after passing between the two halves of the ganglion 
(g7). Muscles (m#s/) are attached to the two divergent ends. 
The mesothoracic furca, which is formed in exactly the same 
manner, is seen in Fig. 86 f, where it passes between the 
commissures (cw); its connection with the muscles is seen 
ENE 
A frontal section (Fig. 91) shows the structure of the meso- 
and meta-thoracic ganglia after they have become loosened from 
the surface ectoderm. The longitudinal (cz) and cross-com- 
missures (¢ cm) are clearly seen as white Punktsubstanz sepa- 
rated from the ganglionic cells by the inner neurilemma (2777). 
The outer neurilemma (0777) is also developed, as are also the 
two main nerve trunks (z!}, 2”), the anterior of which bifurcates 
(z) while leaving the ganglion. 
The separation of the nervous system from the integumen- 
tary ectoderm progresses from before backwards. The two 
brain masses separate first. The first segment becomes very 
small and possibly disappears. The three segments of the 
optic ganglion are invaginated and pushed under the optic 
plate in a manner which I believe to be similar to that de- 
scribed by Patten (39) in Aczdzus, though I have not followed 
the details of the process. The frontal ganglion is formed as an 
unpaired thickening of the dorsal wall of the wsophageal ecto- 
