1917-18.] Germ-Band in the Egg of the Holly Tortrix Moth. 161 
the boundaries of the ventral plate on the dorsal side of the egg, but on 
the ventral side the embryonic tissue still forms part of the blastoderm. 
Later on the cells of the amnion take on the form of a flattened epithelium 
(see figs. 7 c, 8 c, 9c). 
Until the formation of the ventral plate, eggs removed from the leaf and 
kept under microscopical observation appear homogeneous, and exhibit little 
indication of the internal changes which have taken place ; but from this 
time onwards many of the changes can be followed in the living egg, the 
observation of which is in some particulars of more valup than the study of 
sections. I have thought it advisable to show three parallel series of figures 
demonstrating, firstly (figs. 6a- 9a), the appearances seen in the living egg, 
which give the best idea of coincident changes in the yolk ; secondly, the 
corresponding stages exemplified by embryos removed from the egg and 
isolated from the yolk ; and, thirdly, the same stages in transverse section. 
In the living egg one may watch the partition of the yolk by the 
inward movement of the germ-rudiment. This latter itself is only dimly 
seen except at its folded lateral extremities, which stand out as clearly 
defined curves, because here only the denser yolk does not obscure the 
translucency of the embryonal tissue (fig. 6a).* This stage was reached 
in from 12 to 18 hours after oviposition, according to temperature. 
Gradually the embryo changes, until the outline seen in fig. 7 a is 
assumed. Here again the clear zone is due to the absence of yolk, where 
the embryonic epithelium is closely doubled over. A reference to fig. 76 
will give an explanation of this appearance. It is seen that the germ- 
rudiment is rolling inwards so that all its edges approach one another, 
while there is a certain amount of elongation in the direction of the long 
axis of the egg. 
The interval between this and the stage shown in fig. 6a was 2-4 hours. 
Fig. 8a depicts the next change observable in the living egg. This 
figure was drawn from the same egg as fig. 7a, after an interval of three 
hours. The anterior and posterior masses of yolk, parted by the falling 
inwards of the germ-rudiment, are reuniting. This union is accomplished 
very slowly, with an amoeba-like creeping movement of the extremities 
of the two masses towards each other, a movement which gives the 
impression of great viscidity. It also begins to become evident that the 
entire vitellus is at this time breaking up to form “ yolk-balls,” owing to 
* These clear zones often exhibit a faint transverse striation, because, owing to the 
flexure of the embryonal epithelium, the long axis of its component cells is here presented 
to the observer. This appearance has led Graber and others to describe the ventral plate 
as surrounded by a “ striated border.” 
VOL. XXXVIII. 
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