90 CERTAIN PARASITIC INSECTS. 
is called the “ visceral membrane” (Fig. 17, db). Melnikow 
remarks that 
‘In all the insects whose embryology has been studied, and in which 
the ventral primitive streak is developed, neither does the amnion nor the 
visceral membrane take any part in building up the body of the embryo, 
since they are provisional structures in a peculiar sense of the word. 
Quite different relations exist in the lice. The origin of the embryonal 
membranes of the louse occurs at the time of the formation of the prim- 
itive streak. The thickened blastoderm of the end of the egg on which 
the hairs are situated folds in, and this fold is the beginning of the primi- 
tive streak and of the visceral membrane. The layer of this fold facing 
the ventral side of the egg, is transformed into the visceral membrane, 
while the other layer, opposite to the other side of the egg, becomes thick- 
ened and forms the primitive streak. The remaining portion of the 
toderm, with the exception of the primitive streak, which forms the fore- 
head (in the more extended sense of the word) consists of the so-called 
amnion 
In Sobtradiatiection to those insects [Simulium, Chironomus, Donaci 
and Phryganidz] in which a ventral primitive streak is developed, neither 
do the amnion nor visceral membrane form a capsule surrounding the con- 
tents of the egg. "The amnion is intimately connected with the cephalic 
portion of the embryo as also with the visceral membrane. This latter 
is connected only with the abdominal part of the primitive streak, and 
the edges of the side, i. e. the continuation of the amnion. In opposition 
to those above-mentioned insects which have a ventral primitive streak, 
bryonal membranes of insects, are in direct relation to the mode in which 
the primitive streak is formed. It seems, therefore, that the mode of 
origin of the primitive streak, or its position in relation to the yolk is 
concerned in the above-mentioned differences of the embryonal mem- 
branes.* 
s. f 
* Melnikow does not consider, as his fellow countryman, Metznikow, E that the 
embryonal membranes of insects are homologous with those of vertebra iae e says, 
*the mode of origin in ko vertebrates is the same. The formation ay visceral 
rane and amnion of insects ga L differe ue ‘groups, with pened dn d 
formation of the sation streak. mbryonal membranes of vertebrates have a 
certain relation to the allantois, AY ihe pce membranes of insects are corre- 
lated 
on ra 
brates; but in insects differences arise, which become noticeable in the —— of the 
primitive streak in relation to the yolk. Finally, these embryonal membranes in all 
vertebrates are d but in insects this is not the case. They are coii 
only in those which have a ventral awe streak,” (Melnikow). We see, therefore, 
that r the Mme ion of the egg, great and radical differences exist 
between the e eggs of vertebrates and articulat es, and e ven | between different groups of 
thelatter. lstat tt 
E 
