544 EMBRYOLOGY OF THE TURTLE: Part IIL. 
relations of which will presently be pointed out more satisfactorily. The subsidiary 
layer partakes also of the curvatures of the body, following closely its lower sur- 
face, as formerly (PI. 9e, fig. 4 and 4a, n, , 0'); but, imstead of bending up with 
the amnios over the head, the sides, and the tail, it extends almost in a direct 
line (Pl Qe, fig. 4 and 4a, a’, a‘, and Pl. 11, fig. 5a, a*) to the outer edge of 
the area pellucida, (Pl. 9e, fig. 4, d, 4a, d,) and there, meeting the germino-amni- 
otic layer, (Pl. 9e, fig. 4, a, and 4a, a,) follows it closely all over the yolk mass. 
This conduct of the subsidiary layer causes the embryo to appear as if winged, 
(Pl. 11, fig. 5a, @’;) when viewed from below. Ever since the albumen began 
to filtrate into the yolk sac, the subsidiary layer (Pl. 9e, fig. 1, o') has been 
erowing thinner; yet not rapidly, but only at such a rate that it is now (PI. Ye, 
fiz. 4, 4a, o') reduced to about three fifths of the thickness it had in the begin- 
ning. 
Making another advance of from ten to twelve days, we come to an embryo 
in which the amnios, both in its cephalic and caudal part, has progressed pretty 
tar up, “on the backs (Pk 22;\figs 1, <at,ca?;. Pliw Od, figs, a}, a7) Dhes primitive 
furrow has become a deep channel, (Pl. 12, fig. 1, e, Ja, e, e1; Pl. 9d, fig. 1, e, 
e',) forming, at the head, quite a large cavity, (Pl. 9d, fig. 1, e', €%) closed at 
one point (e?) by the arching over and uniting of its walls. Along the back, 
however, this gutter is still open (Pl. 9e, fig. 5, e; Pl. 12, fig. 1, e; Pl. 24, fig. 
13,%¢, fig U8ajye)'; ats lower floore(Ple:9d).. fig; 4,” eis Bias 9e) ties o5ui2) as 
broadened, and the whole is curved upwards into a round, cylindrical form, the 
thickness of which is considerably increased; but its edges, (Pl. 9d, fig. 1, e, 6; 
Pl. Ye, fig, 5, e,) although considerably turned inwardly, remain at a slight dis- 
tance apart, and are still continuous by a sudden reduplication with the imme- 
diate and more peripheric part of this layer, (Pl. 9e, fig. 5, p,) which has_ thick- 
ened also, but thins out toward the area pellucida (¢c). At the posterior half of 
the embryo, this furrow, which may now be recognized as the initiatory phase in 
the formation of the spinal marrow, and of its enlargement, the brain, has not 
become so deep nor so narrow, (Pl. 9d, fig. 1, e; Pl. 12, fig. 1, e; Pl. 24, fig. 
3, e, 13a, e,) but is yet a very broad channel, which gradually grows shallow 
backwards, and its sides become less elevated, till its lower floor is continuous, 
in the same horizon, with the space about it; so that a considerable part of the 
caudal portion (PI. 9d, fig. 1, e4) of the embryo is not yet endowed with a 
special spinal nervous system. 
The broad band (Pl. 9e, fig. 3, f1, fig. 3a, f1, f°, ete.) mentioned in former 
pages, which separates from the subsidiary layer, (o',) has now (Pl. 9d, fig. 1, 77; 
Pl. 12, fig. 1, f1; Pl 24, fig. 13, *) become very much thickened centrally, 
but thins out laterally, and presses closely against the floor of the spinal tube 
