. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine . re prominent and persistent the yelk-sac, the less so Fig. 80.—Diagrammatic dorsal view of an embryo rab-bit with its membranes at the stage of nine so-mites (Hadclon, after Van Beneden and Julin).aly allantois, showing from behind the tail foldof the embryo; am, anterior border of true am-nion ; a. v, area vasculosa, the outer border ofwhich indicates the farthest extension of themesoblast; hi, blastoderm, here consisting only ofepiblast and hypoblast; o. m. v, omphalomesen-teric or v
. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine . re prominent and persistent the yelk-sac, the less so Fig. 80.—Diagrammatic dorsal view of an embryo rab-bit with its membranes at the stage of nine so-mites (Hadclon, after Van Beneden and Julin).aly allantois, showing from behind the tail foldof the embryo; am, anterior border of true am-nion ; a. v, area vasculosa, the outer border ofwhich indicates the farthest extension of themesoblast; hi, blastoderm, here consisting only ofepiblast and hypoblast; o. m. v, omphalomesen-teric or vitelline veins ; p. am, proamnion ; pi,non-vascular epiblastic villi of the future placen-ta ; s. t, sinus terminalis. 80 COMPARATIVE PHYSIOLOGY. the allantois, and vice versa; they are plainly supplementaryorgans. The Allantoic Cavity.—The degree to which the various em-bryonic membranes fuse together is very variable for differentgroups of mammals, including our domestic species. In ruminants, but especially in solipeds, the allantois as itgrows spreads itself over the inner surface of the subzonal. Mm- Fig. 81.—Embryo of dog. twenty-five days old. opened on the ventral side. Chestand ventral walls have been removed, a, nose-pits; b, eyes; c, nnder-jaw (firstgill-arch); d, second gill-arch; e,f, 0, h, heart (e, right,/, left auricle; g, right, h,left ventricle); i, aorta (origin of); kk, liver (in the middle between the two lobesis the cut yelk-vein); /, stomach; m, intestine; n, yelk-sac; o, primitive kidneys;p, allantois; q, fore-limbs; h, hind-limbs. The crooked embryo has been stretchedstraight. (Ilaeckel, after Bischoff.) membrane, often spoken of as the chorion, while it alsocovers, though capable of easy detachment, the outer surface ofthe amnion; and thus is formed the allantoic cavity. The por-tion of the allantois remaining finally within the foetus becomesthe bladder, which during embryonic life communicates by itscontracted portion (urachus) with the general
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