OPMEXT OF THE CHICK SECOND PERIOD. 321 



brane, however, are so far in opposition to the arteries, that 

 whilst these are directed 

 transversely towards the si- 

 nus terminalis (fig. 346, /, 

 f) y those run parallel with 

 the long axis of the embryo ; 

 one interior, larger vein ly- 

 in"; on the left (figs. 346, g, 

 , to which comes a 

 second, smaller, often scarce- 

 ly perceptible one, situated 

 on the right, and either one 

 or two superior veins (figs. 

 346, (t, (/, 339, k 1 } bringing 

 the blood from the vascular 

 area to the heart. The sys- 

 tem of the venae cavae is 

 evolved in the body of the 

 embryo at a still earlier pe- 



Fig. 348. Yolk of the hen's egg, 

 of the natural size, but flattened 

 through loss of support, at the be- 

 ginning of the third day of incubation, 

 exhibiting the earliest traces of the 



circulation. a, Vitellus ; 6, embryo ; 

 c, c, arteries of the blastoderma ; d, d, 

 veins of the blastoderma ; e, e, sinus 

 terminalis. 



riod than the arterial sys- 

 tem, and the portal system 

 is distinctly separated on the 

 fourth day, and ramifying 

 in the liver. The circulation upon the germinal membrane is, 

 therefore, a vitellicular circulation ; the blood courses from the 

 embryo through the two arteriae vitellinae s. omphalo-mesen- 

 tericae (fig. 346,/,/), to the sinus terminalis or vascular circle, 

 which on the fourth day appears quite full of blood ; from this 

 the blood is returned to tlie heart through the four venous 

 trunks the venae vitellinae s. omphalo-mesentericse (fig. 346, 

 g, ff, y). The smallest arteries and veins also communicate 

 with one another by their most delicate extremities, and form 

 a beautiful rete with rhomboidal-shaped meshes. 



[ 491 . There is a very peculiar formation belonging to the 

 foetus alone, and having a temporary or transitory character, 

 which must now be mentioned, namely, the Wolffian bodies, 

 corpora Wolffiana, or primordial kidneys. These bodies 

 are a product of the vascular membrane, though the serous 

 layer would also seem to have some share in their formation. 

 They make their first appearance in the second half of the 

 third day, as a pair of narrow but thick striae, which sprout 



