Fig. 3. The yolk has now become completely enveloped by the blastoderm. The arterial ring has increased in size. The venous ring has vanished, owing to the complete enclosure of the yolk by the blastoderm. The point where it existed is still indicated (y) by the brush-like termination of the main venous trunk in a number of small branches.
Fig. 4. Diagrammatic projection of the vascular system of the yolk sac of a somewhat older embryo.
The arterial ring has grown much larger and the portion of the yolk where no vessels exist is very small (x). The brush-like termination of the venous trunk is still to be noticed.
The two main trunks (arterial and venous) in reality are in close contact as in fig. 5, and enter the somatic stalk close together.
The letter a which points to the venous (blue) trunk should be v and not a.
Fig. 5. Circulation of the yolk sac of a still older embryo, in which the arterial circle has ceased to exist, owing to the space outside it having become smaller and smaller and finally vanished.
[177] Quart. Journ. Micr. Science, Oct. 1874. [This Edition, No. V.]
[178] The part of the brain which I have here called mid-brain, and which unquestionably corresponds to the part called mid-brain in the embryos of higher vertebrates, becomes in the adult what Miklucho-Maclay and Gegenbaur called the vesicle of the third ventricle or thalamencephalon. I shall always speak of it as the mid-brain.
[179] For a fuller discussion of this question vide Self, “A comparison of the early stages of development in vertebrates.” Quart. Journ. of Micr. Science, July, 1875. [This Edition, No. VI.]