, 
CERTAIN PARASITIC INSECTS. 91 
Again, looking at the louse’s egg and its germ (Fig. 17) 
we see the amnion (am) surrounding the yolk mass, and the 
visceral membrane (db) within, partially wrapping the rude 
form of the embryo in its folds. The head (vk) of the em- 
bryo is now directed towards the end of the egg on which 
the hairs are situated ; afterwards the embryo revolves on its 
axis and the head lies next to the opposite end of the egg. 
Our embryo previous to this important change of position 
may be compared with the embryo of the dragon fly (Figs. 
Fig. 18. Fig. 19. 
i I. IL IH. IV. V. VI. VI. 
Embryo of the Dragon-fly, side 
View; I. antennze; Ir. mandibles; rrr. 
maxilla; 1v. second maxille (lab- 
jum); v.- vir. legs. 
Fig. 19, ventral view of the same. 
18,19). Eight tubercles bud out from the under side of the 
head, of which the foremost and longest are the antenne (as), 
those succeeding are the mandibles, maxille, and second 
maxille, or labium. Behind them arise six long, slender 
tubercles forming the legs, and the primitive streak rudely 
marks the lower wall of the thorax and abdomen, not yet 
formed. Figure 20 represents the head and mouth parts of 
the embryo of the same louse ; vk is the forehead, or clypeus; 
ant, the antenna; mad, the mandibles; max, the first pair 
at first the eggs of all animals, as well as the early stages of the embryo, are alike, have 
not regarded the important differences presented at the first sketching out of the em- 
ryo. The great differences between the two branches of vertebrates and articulates 
arise before the most rudimentary form of the embryo is indicated; indeed it may be 
said with truth, at the first beginnings of life. Those also who indulge in glittering 
iti . and the p 
neralities regarding the identity in th e eggs of animals, and th 
topl ic matter ich they a: mposed, should also take into account the radic: 
differences of the mode of action of this lasm (i. e. egg-contents, yolk and albu- 
men) in the eggs of vertebrates and insects at the dawn of life, whether they be due to 
the “vital force,” or to some chemical force conserved and metamorphosed into a 
ife-giving power, 
