Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 5ol 
the mode of origin of whatever other organs may make their appearance in these 
later stages of embryonic growth. 
In a slightly farther advanced stage, (Pl. 18a, fig. 13,) we find the amnios 
(a!) has nearly closed over; the nervous system is about the same as in the last; 
the eyes (/) have each a distinct crystalline lens (Pl. 24, fig. 8, 2°); the ears 
are more trumpet-shaped, (Pl. 18a, fig. 13, 4) and extend deeper into the head; 
and the dorsal vertebre are perhaps more separated from each other. In this 
embryo we have been enabled to trace very distinctly the connection of the 
forked vessel, (7) which opens into the heart, (/,) with the converging vessels 
mentioned above, (see Pl. 12, fig. 7, 7-7; Pl. 14, fig. 11,) which come from the 
outer edge of the area vasculosa. From the anterior part of the heart, the ven- 
tricle, (Pl. 18a, fig. 13, 4%) a large vessel (/')* arises and passes along just below 
the branchial fissures (mm, m) towards the head. From the dorsal side of this large 
vessel, the aorta, (Pl. 18a, fig. 11, 2)? other small vessels (/7, 17, h*) proceed 
between the branchial fissures (7) on each side of the head upwards, and join 
another large vessel, the dorsal artery (j?). The dorsal artery follows closely against 
the median line of the vertebral layer above, till it reaches the posterior end of 
the body. [See a little older embryo, (Pl. 18a, fig. 14, 7°) to trace its course as 
seen in profile] When the embryo is viewed from below, (Pl. 18, fig. 7,) the 
dorsal artery may be seen giving off to the right and to the left in the abdom- 
inal region, numerous vessels, which at once spread and ramify through the vas- 
cular area (p. 538). These numerous and minute vessels, the omphalo-meseraic 
arteries, have a general trend towards the circular channel spoken of above, (p. 
549,) the so-called vena terminalis, into which they empty. On each side of the 
head the yena terminalis converges and joins the forked vessel, (Pl 18a, fig. 13, 
i, 7) which is connected with the posterior end, the auricle, of the heart. Thus 
we have a perfect circuit in the circulation of the blood. At the outset, the 
heart, the first part of the vascular system in which a fluid may be seen in 
motion, sends the blood forward, through the arteries of the branchial fissures, to 
the dorsal artery ; the dorsal artery sends off currents into the area vasculosa ; 
these currents, the omphalo-meseraic arteries, empty into the vena terminalis; and 
the vena terminalis returns the blood to the heart, through the forked vessel, the 
vena afferens. 
There are also, within the body, circuits of blood of lesser extent than that 
1 Pl. 18a, fig. 13, 4°, 44. The dotted line, extend- of the aorta which is just below the longest branchial 
ing in the original drawing from these letters to the fissure, (m,) which runs from the ear (2) downwards. 
parts they designate, has been accidentally omitted ? This figure, although representing a little older 
here. From h® the dotted line should extend to the phase, will serve to show the direction of the vessels 
dark hole, in the nearest part of the heart, to that part in this region of the body. 
