350 WHEELER. (Vor. III. 
contraction continues towards the median dorsal line and towards 
the cephalic pole. In Fig. 11 the dorsal organ has moved half- 
way up the dorsal surface. Its darker color in stained embryos 
is due to the fact that the serosal cells have become deeply 
columnar with a consequent approximation of their large nuclei 
to one another. The embryo, besides increasing in size, has 
undergone a change in position. Its tail now lies at the caudal 
end of the egg. Notwithstanding the embryo’s growth in length, 
its head lies much lower than in the preceding stage (Fig. 10). 
The body wall (0) is still distinct, the zone of sporadic amni- 
otic (?) nuclei (x) has increased in breadth. As in the preceding 
stages, these nuclei are most closely aggregated near the edge 
of the advancing body wall. In the next stage (Fig. 12) the 
dorsal organ has reached the cephalic end of the yolk and bulges 
out like a large hood. The body walls of the embryo have 
nearly enveloped the yolk at the caudal end. 
The next change takes place very rapidly. The stage repre- 
sented in Fig. 12 is attained towards the end of the sixteenth 
day. By the seventeenth day the walls have closed in the 
median dorsal line, and the embryo has grown in length to such 
an extent as to bring its head to the cephalic pole. The dorsal 
organ has been shut in by, and lies immediately below, that por- 
tion of the body wall, which will form the tergum of the pro- 
thorax. On entering the yolk the cells of the dorsal organ begin 
to disintegrate. Two of the stages in the formation and disso- 
lution of the dorsal organ are represented in Figs. 50 and 51, 
both from longitudinal sections, the former being sagittal, the 
latter frontal. 
Figure 50 represents a section through the centre of the thick- 
ened mass of serosal cells. The deeply stained nuclei are seen 
crowded together in the inner ends of the cells, the contours of 
which are rendered papillose apparently by the pressure of the 
nuclei against the cell walls. The outer ends of the elongated 
columnar cells form a thick layer of granular protoplasm con- 
siderably depressed at 0. This depression is equivalent to the 
tubular cavity in the dorsal organ of //ydrophilus. 
In Fig. 51 the large lump of cells has become engulfed in the 
yolk. The body wall has closed over it, and the heart (cc) has 
formed between it and the ectoderm. The large deeply staining 
nuclei (zz) are seen in the various stages of active degeneration, 
