No. 2.] BLATTA AND DORYPHORA. 349 
In Blatia the formation of the nervous system in its earlier 
stages cannot be clearly seen from the exterior. The same 
holds true of the small tracheal invaginations, though several 
pairs, especially those of the thorax and basal abdominal rings, 
may be seen on the pleuree in good preparations before revolu- 
tion. Still they are so much less distinct than in Doryphora 
that I have given them little attention. 
The peculiar phenomena of revolution are hurried through by 
the embryo from the beginning of the sixteenth to the end of 
the seventeenth day. Several successive stages in the process 
are represented in the woodcuts (Figs. 9 to 13). When fifteen 
days old (Fig. 9) the embryo still occupies the middle of the 
ventral surface of the egg, the distance from the head to the 
cephalic end of the yolk being almost equal to the distance of 
the tail from the caudal end of the yolk. The amnion and 
serosa (as) still envelop the embryo, though they have become 
much attenuated. By the end of the fifteenth or the beginning 
of the sixteenth day, the envelopes rupture, an irregular slit 
being formed down the median ventral line. The amnion now 
appears to undergo degeneration, at least in part, while the 
serosa is drawn back from both sides by a contraction of the 
protoplasm of its cells, the large nuclei of which make it easy 
to trace all the steps in the formation of the dorsal organ. 
Soon after the rupture of the envelopes the embryo and egg, 
when seen from the side, resemble Fig. 10. The embryo stands 
out free from its envelopes on the yolk; the edges of its dorsad 
growing walls (4) are distinctly marked. Near these, on the sur- 
face of the egg, are seen a number of scattered nuclei, which are 
of the same size as the nuclei of the cells forming the embryo. 
These I take to belong to the portion of the amnion which has 
become folded back on the yolk and forms a zone (7) extending 
the whole length of the yolk in contact with the dorsad growing 
body wall. Next to this zone lies another zone, which is bounded 
by the distinct edge of the serosa (s), and which I regard as a 
portion of the yolk left bare, as no nuclei are to be found on its 
surface. Besides the ventrodorsal contraction in the substance 
of its anterior edge (s), the serosa contracts in an antero-poste- 
rior direction, thus producing the constriction seen at y in Fig. 
10. The rounded and projecting lump formed at the caudal 
pole is the beginning of the dorsal organ (Fig. 10 @. 0). The 
