974 FORMATION OF THE AMNION AND THE ALLANTOIS. 



FORMATION OF THE AMNION AND THE ALLANTOIS. 



During the process of folding-off of the embryo there results, 

 first (at the end of the second day in the chick) in front of the head, a 

 foldlike elevation, consisting of epiblast and the outer layer of meso- 

 blast. This is reflected like a cowl to form the head-fold for the cephalic 

 portion of the embryo (Fig. 376, VI, A). Later and more slowly there 

 develops the caudal fold from behind, and, finally, also between these 

 two the lateral folds are formed (Fig. 376, III, A). As all of these 

 folds tend toward the back of the embryo they finally grow together 

 and form the amniotic sac (in the chick on the third day). There 

 is thus formed about the embryo a cavity that becomes filled with 

 amniotic fluid. Also in mammals the amnion develops early and 

 in the same way as in birds (Fig. 376, VII, A). From the middle of 

 pregnancy the amnion lies in immediate contact with the chorion, 

 with which it is united by a layer of gelatinous tissue (tunica media). 



Both the amnion and the allantois develop only in mammals, birds, and rep- 

 tiles, which therefore are designated also amniota, while the lower vertebrates, 

 the anamnia, are without these structures. The amniotic liquor is a clear, serous 

 alkaline fluid, having a specific gravity of from 1002 to 1028. It contains, in 

 addition to epithelium, lanugo-hairs and from to 2 per cent, of fixed solids. 

 The latter comprise albumin (from T V to $ per cent.), mucus, globulin, a body resem- 

 bling vitellin, some grape-sugar (cow), allantoin, urea, ammonium carbonate (prob- 

 ably transformed from urea) , sometimes lactic acid and kreatinin, calcium sulphate 

 and phosphates, and sodium chlorid. The total amount of fluid at the middle of 

 pregnancy is_ from i to 1.5 kilos; at the end of pregnancy 0.5 kilo. 



The amniotic liquor is of fetal origin, as its presence in birds indicates, and it may 

 be a transudate from the ovular membranes. In mammals the urine of the fetus 

 probably contributes to the accumulation of the fluid in the second half of preg- 

 nancy. In cattle , in which the allantoic and the amniotic fluids remain permanently 

 separate, the first may be regarded as fetal urine, the latter as transudate. In 

 the presence of the pathological condition of hydramnios, also the vessels of the 

 uterine mucosa may secrete serum, especially when there is stasis in the distri- 

 bution of the umbilical vein in the placenta. The amniotic fluid protects the fetus 

 and the vessels of the fetal membranes from external injuries; it affords free 

 movement to the limbs, and thus prevents them from forming adhesions ; finally, 

 it is important during the act of parturition for the dilatation of the mouth of 

 the uterus. The amnion is contractile (in the chick from the seventh day on), 

 from the presence of smooth muscle-fibers that develop in the cutaneous plate 

 (mesodermal portion). Nerves have not been found. 



From the anterior extremity of the hind-gut there grows a vesicular 

 sac, which appears at first as a small double tubercle and then becoming 

 hollow (Fig. 376, VII, a); it projects into the ccelom-cavity. This is 

 the allantois or urinary sac (in the chick before the fifth day; in man 

 during the second week). As a true evagination from the hind-gut, 

 the allantois has two layers: one from the entoblast, and the other from 

 the splanchnopleure. From each side there passes upon the sac the 

 allantoic or umbilical artery, arising from the hypogastric artery, and 

 ramifying upon the surface of the sac. The allantois grows (like a 

 steadily filling urinary bladder) in front of the hind-gut in the abdominal 

 cavity toward the umbilicus, and finally out of this (at the side of the 

 omphalpmesenteric duct), together with its vessels (VII, a), and it ex- 

 hibits different relations in birds and in mammals. 



In birds, the allantois, after passing out at the umbilicus, undergoes excessive 



rth, in a short time lining the entire inside of the shell as a vascular sac. Its 



arteries, at first branches of the primitive aorta, appear with the development 



