Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 5d7 
where the anus originates at a later period, there is a deep, transverse fissure, 
the first indication of the posterior opening of the alimentary canal. 
Another embryo, (Pl. 14, fig. 4,) to all appearances identical in development 
with this, has a considerably larger and much more highly vascular allantois. 
The allantoidian artery (0) is quite large, and the allantoidian vein (¢) has 
already assumed that wavy course which is so characteristic in older phases. 
The abdominal artery and the abdominal vein are here very conspicuous along 
the whole length of the body, even to the end of the tail. 
The embryo of another species, Nanemys guttata, (Pl. 16, fig. 6, 6a,) although 
not more developed than the last two, has an allantois still larger than either of 
them. It extends its bulk along the whole abdominal region, from the heart to the 
tip of the tail, and even beyond. The median constriction, which we have already 
pointed out in a much younger phase, (p. 555; Pl. 24, fig. 15, n°.) is here very 
conspicuous, yet does not trend in the direction of the longitudinal axis of the 
body, as at the time of its earliest appearance, but is twisted so as to run 
obliquely across its former path. This change corresponds with the alteration in 
the position of the allantois, which, instead of lying symmetrically across the 
embryo, rests with one part of its constricted bulk next to the head, and the 
other part in the caudal region. This is the first indication we have of the 
tendency of this organ to spread over the surface of the animal. After noticing 
that the line of constriction of the allantois is occupied by the main vessels, 
arteries and veins, and referrmg to the youngest phase in which this constriction 
is visible, but without bloodvessels, it becomes evident that the path of the allan- 
toidian arteries and veins is marked out almost from the time of the origin of 
the allantois. 
We have already pointed out the change in the attitude of the embryo (p. 541, 
and 554; Pl. 13, fig. 2) from a vertical to a horizontal position, with its left 
side downwards; but will refer to it again now, in connection with a correspond- 
ing change which the allantois has assumed im its position. Inasmuch as the right 
half of the allantois cannot expand laterally, with reference to the embryo, on 
account of the ege shell, which is closely above, it must take another direction, 
and consequently the left half also is moved from its former position. It is this 
change in the direction of its expansion that has twisted the whole allantois upon 
its axis. Here, too, the sinking of the area pellucida becomes conspicuous, not 
only in consequence of its having fallen below the general level of the vascular 
area, but also because of its bearing upon the expansion of the allantois, to 
which it gives place. The boundaries of this depression are marked by a_sud- 
den bending upwards of the omphalo-meseraic bloodvessels. The bloodvessels of 
the allantois are very numerous, and anastomoze with each other by a multitude 
