174 General Notes. [ February, 
inside of the ovarian-tube cells (of) remain as an appendage to 
the posterior pole of the egg. 
4. Formation of Germ-streak—Energetic cell multiplication at 
the posterior pole causes a new invagination at that place (Fig. 
One side of the invaginated part is of thick cells, this is the 
germ-streak (gs) (ventral plate of Balfour), and ultimately the em- 
bryo ; the other side of the invaginated part is of thin cells, this 
becomes the amnion (am). The blastoderm remains thin, except 
where it adjoins the outer extremity of the germ-streak, where it 
is thickened so as to form lateral plates (44). Thus embryo and 
amnion are both buried in the center of the egg; the embryo 
bends ventraliy, the abdomen curving round so as to approximate 
to the posterior pole, and to have appendages and amnion within 
its bend. The head end is at the place of invagination, lateral 
plates codperating with the extremity of the germ-streak towards 
the formation of head and brain. 
The central position of the embryo is characteristic of the 
lower orders of insects, which are therefore termed entoblastic. 
. These include Hemiptera, Orthoptera and probably Thysanura. 
AP The higher orders, as Hymenoptera, 
se Lepidoptera, Coleoptera, Diptera and 
perhaps Neuroptera, are ectoblastic, 
having a ventral plate formed on the 
surface, afterwards sinking slightly 
under the blastoderm, and having the 
anterior pole of the egg the more ac- 
tive. The ectoblastic condition seems 
to be a case of anticipation of changes 
Ją Which must afterwards be encountered 
SI by the inferior forms (see section 14 
j - below). , 
. The blastoderm being now thin 
uD becomes the serous tunic around the 
o mo whole egg. 
dei oe wan Dn ea Genital Cells —A few large cells 
in the egg. arise at an early stage inside the blas- 
| toderm, near the place of invagination 
(ge). These come to be ultimately received into the embryo 
along with the pseudo-vitellus, and become paired generative or- 
gans, groups of ovarian tubules. | j 
7. Germinal Layers—The germ-streak divides into an outer 
and an inner layer, each cell dividing into an outer and inner 
part. Thus are formed ectoderm and mesoderm (ec, ms in Fig. 3; 
also indicated by the dotted line in Fig. 2). The few cells 
formed in the center of the egg are the only representatives of 
endoderm. (In ectoblastic insects the mesoblast is formed by an 
infolding of ectoderm. 
8. The appendages next show themselves, those of the head 
ae 
