. A laboratory manual and text-book of embryology. Embryology. EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 85 Body-stalk fore the development of the fore-gut or hind-gut. In Peter's embryo the amnion, chorion and yolk-sac are present but not the allantois (Fig. 72). In an embryo mm. long, described by Von Spee (Fig. 73 A, B), there is no hind-gut, but the allantoic diverticulum of the entoderm has invaded the mesoderm of the body- stalk. This embryo, seen from the dorsal side with the amnion cut away, shows a marked neural groove and primitive streak. In front of the primitive knot a pore is
. A laboratory manual and text-book of embryology. Embryology. EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 85 Body-stalk fore the development of the fore-gut or hind-gut. In Peter's embryo the amnion, chorion and yolk-sac are present but not the allantois (Fig. 72). In an embryo mm. long, described by Von Spee (Fig. 73 A, B), there is no hind-gut, but the allantoic diverticulum of the entoderm has invaded the mesoderm of the body- stalk. This embryo, seen from the dorsal side with the amnion cut away, shows a marked neural groove and primitive streak. In front of the primitive knot a pore is figured leading from the neural groove into the primitive intestinal cavity, hence called the neurenteric canal. The fore-gut and head-fold have formed at this stage and there are branched chorionic villi. A reconstruction by Dandy of Mall's embryo, about 2 mm. long with seven pairs of seg- ments, shows well the embryonic appendages (Fig. 74). The fore- and hind-gut are well developed, the amniotic cavity is large, and the yolk-sac still communicates with the gut through a wide open- ing. The allantois is present as a long curved tube somewhat di- lated near its blind end and em- bedded in the mesoderm of the body-stalk. As the hind-gut de- velops, the allantois comes to open into its ventral wall. A large umbilical artery and vein are present in the body-stalk. In an embryo of 23 somites mm. long, described by Thompson, the allantois has elongated and shows three irregular dilatations (Fig. 75). A large cavity never appears distally in the human allantois as in Ungulates. When it becomes included in the umbilical cord its distal portion is tubular and it eventually atrophies. That part of the allantois extending from the umbilicus to the cloaca of the hind-gut takes part in forming the urogenital sinus, the bladder and the urachus, a rudiment extend- ing as a solid cord from the fundus of the bladder to the umbilicus. According to Felix, the allantois forms only the urachu
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectembryology, bookyear1
