The blastodermic vesicle during the early stages of development lies freely in the uterus; and no non-vascular villi, similar to those of the Dog or the Rabbit, are formed before the appearance of the allantois. The blastodermic vesicle has at first the usual spherical form, but it grows out at an early period, and with prodigious rapidity, into two immensely long horns; which in cases where there is only one embryo are eventually prolonged for the whole length of the two horns of the uterus. The embryonic area is formed in the usual way, and its long axis is placed at right angles to that of the vesicle. On the formation of an amnion there is formed the usual subzonal membrane, which soon becomes separated by a considerable space from the yolk-sack (fig. 153). The yolk-sack is, however, continued into two elongated processes (_yk_), which pass to the two extremities of the subzonal membrane. It is supplied with the normal blood-vessels. As soon as the allantois appears (fig. 153 _all_), it grows out into a right and a left process, which rapidly fill the whole free space within the subzonal membrane and in many cases, _e.g._ the Pig (Von Baer), break through the ends of the membrane, from which they project as the diverticula allantoidis. The cavity of the allantois remains large, but the lining of hypoblast becomes separated from the mesoblast, owing to the more rapid growth of the latter. The mesoblast of the allantois applies itself externally to the subzonal membrane to form the chorion[91], and internally to the amnion, the cavity of which remains very small. The chorionic portion of the allantoic mesoblast is very vascular, and that applied to the amnion also becomes vascular in the later developmental periods. [91] According to Bischoff the subzonal membrane atrophies, leaving the allantoic mesoblast to constitute the whole chorion. [FIG. 153. EMBRYO AND FOETAL MEMBRANES OF A YOUNG EMBRYO ROE-DEER. (After Bischoff.) _yk._ yolk-sack; _all._ allantois just sprouting as a bilobed sack.] The horns of the yolk-sack gradually atrophy, and the whole yolk-sack disappears some time before birth. Where two or more embryos are present in the uterus, the chorions of the several embryos may unite where they are in contact. From the chorion there grow out numerous vascular villi, which fit into corresponding pits in the uterine walls. According to the distribution of these villi, the allantois is either diffused or polycotyledonary. [FIG. 154. PORTION OF THE INJECTED CHORION OF A PIG, SLIGHTLY MAGNIFIED. (From Turner.) The figure shews a minute circular spot (_b_) (enclosed by a vascular ring) from which villous ridges (_r_) radiate.] [FIG. 155. SURFACE-VIEW OF THE INJECTED UTERINE MUCOSA OF A GRAVID PIG. (From Turner.) The fig. shews a circular non-vascular spot where a gland opens (_g_) surrounded by numerous vascular crypts (_cr_).]