TRANSACTIONS OF SECTION D. 425 
During the three days previous to hatching, the dorsal vessel increases its 
pulse from about thirty to the minute to between eighty and one hundred. Just 
before hatching, a cephalic heart appears in the posterior head region. At first 
small and only pulsating intermittently, it rapidly increases in size. The 
pressure thus caused forces the pedicel to break away from the egg, whereupon 
the nymph flows easily and quickly out of the egg-shell. It emerges swathed 
in an outer skin or sheath, which has been called by Pierre the ‘amniotic cover- 
ing. This is shown to be a non-cellular chitinous cuticle, not related to the 
amnion in any way. It represents, in fact, the first moult of the larva. The 
swathed stage may be termed the pro-nymph. 
The pro-nymph stage lasts only a few seconds. The cephalic heart increases 
enormously, and is seen to consist of two large chambers, an auricle and a 
ventricle, which pulsate regularly at about thirty beats to the minute, and 
appears to be pumping liquid, probably blood, into the head. The latter swells 
quickly up to twice its original size, and thus the pro-nymphal sheath soon splits 
down the back of the head and thorax, and the young nymph emerges, freeing 
itself from the sheath by a few convulsive struggles. 
The pro-nymphal sheath is seen to be made of very thin transparent chitin, 
and shows the complete larval form, with head, mouth-parts, and legs easily 
seen. It ends posteriorly in a sharp spine, which catches in the broken end of 
the egg, and so forms an anchor during the emergence of the nymph. 
The cephalic heart quickly subsides in the free nymph. Meanwhile, a smaller 
pulsating chamber has appeared between the rectal valves. While the cephalic 
heart is forcing the blood into the head, this rectal pulsating organ appears to be 
pumping water into the rectum. As soon as the nymph is free, its pulsations 
increase to about eighty per minute, and water is violently forced into the 
rectum, so that the whole beautiful branchial basket is quickly distended and 
brought into view. Meanwhile, the tracheal system, which, at the time of 
hatching, only contained air anteriorly to the mid-gut, is seen to be steadily filling 
with air. The air travels slowly down the dorsal tracheal trunks and gradually 
fills the numerous branches, finally entering all the tiny tracheoles of the rectal 
gills. Afterwards, rectal breathing proceeds regularly. 
The young nymph is transparent except for the eyes and the dark plug of 
the mid-gut. It has two sharply pointed cerci, but the superior appendage is 
only rudimentary. In a few hours the nymph darkens all over to dull green or 
blackish. It is suggested that the rupture and atrophy of the amnion described 
by Brandt in the embryology of Odonata is due to the formation of the pro- 
nymphal sheath or cuticle, which forms a close-fitting and far more effective 
protection for the embryo, besides allowing for the early beginning of the process 
of excretion through the formation of a chitinous exoskeleton. 
