Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 539 
” 
may mention in passing, that about this time the “primitive stripe” appears upon 
the oldest conical prominence, (Pl. 11, fig. 3, 4,) next to the older depression of 
the embryonic dise, the cephalic hood; and that the albumen has nearly half filled 
the space underlying the embryo (Pl. 11, fig. 4a). A few more hours bring about 
a considerable change (Pl. 11, fig. 5, and 5a): the sides of the embryo are so 
approximated as to give it the shape of an inverted lyre. // The oldest conical 
prominence has broadened considerably, and is folded under ‘more suddenly ; the 
depression (Pl. 11, fig. 5, w', fig. 5a, a’) of that side is like a narrow, transverse 
furrow broadened at each end, and that on the opposite side (a) has become also 
deeply lunate by the sharp downward folding of the middle line of the body. 
At this time the “primitive stripe” (Pl. 11, fig. 4, 4) extends along two thirds 
of the length of that portion of the embryo which is visible outwardly ; and below 
it is represented by a sharp median ridge, (Pl. 11, fig. 5a, 6; Pl. 9e, fig. 4a, o',) 
which does not belong to the same layer as the primitive stripe, (Pl. 11, fig. 5, 
b,) but is merely forced downwards by its encroachment. The primarily depressed 
end has become not only much more sunk towards the centre of the yolk mass, 
but is bent strongly upon itself (Pl. 11, fig. 5a, at; Pl. 9e, fig. 4, et) and rolled 
inwardly at the sides, (Pl. 9e, fig. 4a, ¢’,) and that portion of the germinal layer 
which forms the depression now embraces it closely, (Pl. 9e, fig. 4, @°,) nearly as 
far up as the level of the exterior surface of the body; but at the opposite end 
the depression is not so deep, and consequently the upward folding is not as 
extensive (Pl. 9e, fig. 4, a’). At this age the embryo hangs over a vast albu- 
minous mass, (Pl. 9e, fig. 4, a/, and Pl. 11, fig. 5b, 5c,) occupying more than one 
half of the yolk sac, the remaining space of which contains the yolk. The changes 
which the dimensions of the yolk and of its membranes undergo in consequence of 
this infiltration of albumen will be fully described in another section. 
The next stage in the development reveals which is the head, and which the 
posterior end, of the embryo. It thus appears that that part of the embryonic 
dise which is first depressed, (Pl. 11, fig. 1, a’,) and which, in the last stage, 
hung so low below the rest of the body, (Pl. 11, fig. 5a, at; Pl. 9e, fig. 4, ¢',) 
is the head, for the development of the brain (PI. 12, fig. la, et) is evidently 
going on here The fold of the amnios arising from this part is therefore the 
so-called “cephalic hood,” and the other, at the opposite end, the “caudal hood.” 
The amnios, folding backward and upward, and thus far closely following (PI. 9e, 
fig. 4, a’, a’, a®) the outer surface of the embryo, has now covered a considera- 
ble portion of the head and back,—inclosing the head as with a closely fitting 
cowl, (Pl. 12, fig. la, a’,) the thickness of which, in profile, resembles a narrow 
1 By mistake in lettering this plate, c! was inserted for e?. A sharp pencil may easily correct this oversight. 
