DEVELOPMENT 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, /, 

 /), those run parallel with 

 the long axis of the embryo ; 

 one inferior, larger vein ly- 

 ing on the left (figs. 346, g, 

 339, k~\ to which comes a 

 second, smaller, often scarce- 

 ly perceptible one, situated 

 on the ri2;ht, and either one 



O * 



or two superior veins (figs. 

 346, g, g, 339, k 1 } bringing 



the blood from the vascular 



area to the heart. The sys- Fi g- 348. Yolk of the hen's egg, 



tern of the venae cavae is f the , ? atura i size ' ^flattened 



, . , , , ,, , through loss of support, at the be- 



evolved in the bodyot the g i nn i ng O f the third day of incubation, 



embryo at a still earlier pe- exhibiting the earliest traces of the 



riod than the arterial sys- circulation. a, Vitellus ; b, embryo ; 



tern, and the portal system c > c > arteries of the blastoderma ; d, d, 



is distinctly separated on the ns f. the blastoderma ; *> ** sj s 



J ., . terminalis. 



fourth day, and ramifying 



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

 therefore, avitellicular circulation ; the blood courses from the 

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

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

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

 the blood is returned to the heart through the four venous 

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

 g, g, g). 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 



